feat(playlist): 歌单数据层 + 加入歌单入口

数据层：
- PlaylistSummary（歌单名/歌曲数/封面），DB 用一条 SQL 取摘要，
  封面取歌单内第一首有封面的歌
- PlaylistRepository.getSummaries() / addSongs()（批量加入并去重）
- 修复 AudioService.playPlaylist：之前把文件路径直接当 URL，
  歌单播放必然失败；现在用 WebDAVService.getFileUrl 构造

入口（视觉为临时实现，等设计稿替换）：
- 文件列表长按 -> "加入歌单" 底部弹层
- 可在弹层里新建歌单并直接加入

真机验证：长按歌曲 -> 新建 "TL" -> 数据库 playlists/playlist_songs 均有记录。
测试：新增 playlist_summary 4 个用例，累计 29 个全绿。

lib/database/song_database.dart

@@ -313,6 +313,26 @@ class SongDatabase {
     return await db.query('playlists', orderBy: 'created_at DESC');
   }
 
+  /// 歌单摘要：歌曲数 + 封面（取歌单内第一首有封面的歌）。
+  /// 列表页只需要这些，不用把每首歌都拉出来。
+  Future<List<Map<String, dynamic>>> getPlaylistSummaries() async {
+    final db = await database;
+    return await db.rawQuery('''
+      SELECT p.id, p.name, p.updated_at,
+        (SELECT COUNT(*) FROM playlist_songs ps
+          WHERE ps.playlist_id = p.id) AS song_count,
+        (SELECT s.artwork_path FROM playlist_songs ps2
+           JOIN songs s ON s.remote_path = ps2.song_id
+          WHERE ps2.playlist_id = p.id
+            AND s.artwork_path IS NOT NULL
+            AND s.artwork_path != ''
+          ORDER BY ps2.order_index
+          LIMIT 1) AS cover_path
+      FROM playlists p
+      ORDER BY p.updated_at DESC
+    ''');
+  }
+
   Future<Map<String, dynamic>?> getPlaylist(String id) async {
     _logQuery('playlists', 'get', args: {'id': id});
     final db = await database;

lib/models/playlist_summary.dart

@@ -0,0 +1,33 @@
+// lib/models/playlist_summary.dart
+
+/// 歌单列表用的轻量摘要（不加载歌单内全部歌曲）
+class PlaylistSummary {
+  final String id;
+  final String name;
+  final int songCount;
+
+  /// 封面：取歌单里第一首有封面的歌
+  final String? coverPath;
+  final int updatedAt;
+
+  const PlaylistSummary({
+    required this.id,
+    required this.name,
+    this.songCount = 0,
+    this.coverPath,
+    this.updatedAt = 0,
+  });
+
+  factory PlaylistSummary.fromRow(Map<String, dynamic> row) {
+    final cover = (row['cover_path'] as String?)?.trim() ?? '';
+    return PlaylistSummary(
+      id: row['id'] as String? ?? '',
+      name: (row['name'] as String?)?.trim() ?? '',
+      songCount: (row['song_count'] as num?)?.toInt() ?? 0,
+      coverPath: cover.isEmpty ? null : cover,
+      updatedAt: (row['updated_at'] as num?)?.toInt() ?? 0,
+    );
+  }
+
+  bool get isEmpty => songCount == 0;
+}

lib/pages/webdav_file_list_page.dart

@@ -7,6 +7,7 @@ import '../controllers/playback_controller.dart';
 import '../database/song_database.dart';
 import '../services/audio_service.dart';
 import '../widgets/song_artwork.dart';
+import '../widgets/add_to_playlist_sheet.dart';
 
 class WebDAVFileListPage extends StatefulWidget {
   final String currentPath;
@@ -435,6 +436,7 @@ class _WebDAVFileListPageState extends BaseState<WebDAVFileListPage> {
           onPressed: () => _toggleFavorite(item.path),
         ),
         onTap: () => _playSong(item),
+        onLongPress: () => showAddToPlaylistSheet(context, songPath: item.path),
       );
     }
   }

lib/repositories/playlist_repository.dart

@@ -2,6 +2,7 @@
 import 'dart:math';
 import '../database/song_database.dart';
 import '../models/playlist.dart';
+import '../models/playlist_summary.dart';
 import '../services/audio_service.dart';
 
 class PlaylistRepository {
@@ -37,6 +38,25 @@ class PlaylistRepository {
     return playlist;
   }
 
+  /// 歌单列表摘要（歌曲数 + 封面），给主页/列表页用
+  Future<List<PlaylistSummary>> getSummaries() async {
+    final rows = await _db.getPlaylistSummaries();
+    return rows.map(PlaylistSummary.fromRow).toList();
+  }
+
+  /// 批量加入歌曲（自动跳过歌单里已有的，避免打乱顺序）
+  Future<int> addSongs(String playlistId, List<String> songIds) async {
+    if (songIds.isEmpty) return 0;
+    final existing = (await getPlaylistSongIds(playlistId)).toSet();
+    var added = 0;
+    for (final id in songIds) {
+      if (existing.contains(id)) continue;
+      await _db.addSongToPlaylist(playlistId, id);
+      existing.add(id);
+      added++;
+    }
+    return added;
+  }
   Future<void> updatePlaylistName(String id, String newName) async {
     await _db.updatePlaylist(id, {
       'name': newName,

lib/services/audio_service.dart

@@ -296,18 +296,19 @@ class AudioService extends ChangeNotifier {
     for (final id in songIds) {
       final dbSong = await _db.getSongByPath(id);
       if (dbSong != null) {
+        final path = dbSong['remote_path'] as String;
         songs.add(Song(
-          id: dbSong['remote_path'] as String,
+          id: path,
           title: dbSong['title'] as String? ?? '',
           artist: dbSong['artist'] as String? ?? '未知艺术家',
-          url: dbSong['remote_path'] as String,
+          url: WebDAVService.instance.getFileUrl(path),
         ));
       } else {
         songs.add(Song(
           id: id,
           title: id.split('/').last.replaceAll(RegExp(r'\.[^.]*$'), ''),
           artist: '未知艺术家',
-          url: id,
+          url: WebDAVService.instance.getFileUrl(id),
         ));
       }
     }

lib/widgets/add_to_playlist_sheet.dart

@@ -0,0 +1,166 @@
+// lib/widgets/add_to_playlist_sheet.dart
+import 'package:flutter/material.dart';
+
+import '../models/playlist_summary.dart';
+import '../repositories/playlist_repository.dart';
+
+/// 把一首歌加入歌单的底部弹层。
+///
+/// 视觉先保持系统样式，等设计定稿后只换这个文件即可，
+/// 调用方（文件列表长按）不用改。
+Future<void> showAddToPlaylistSheet(
+  BuildContext context, {
+  required String songPath,
+}) async {
+  final repo = PlaylistRepository();
+  List<PlaylistSummary> playlists;
+  try {
+    playlists = await repo.getSummaries();
+  } catch (e) {
+    if (context.mounted) {
+      ScaffoldMessenger.of(context).showSnackBar(
+        SnackBar(content: Text('读取歌单失败: $e')),
+      );
+    }
+    return;
+  }
+  if (!context.mounted) return;
+
+  await showModalBottomSheet<void>(
+    context: context,
+    backgroundColor: const Color(0xFF1A1F1E),
+    builder: (sheetContext) {
+      return SafeArea(
+        child: Column(
+          mainAxisSize: MainAxisSize.min,
+          children: [
+            const Padding(
+              padding: EdgeInsets.fromLTRB(20, 18, 20, 8),
+              child: Align(
+                alignment: Alignment.centerLeft,
+                child: Text(
+                  '加入歌单',
+                  style: TextStyle(
+                    color: Colors.white,
+                    fontSize: 16,
+                    fontWeight: FontWeight.w500,
+                  ),
+                ),
+              ),
+            ),
+            ListTile(
+              leading: const Icon(Icons.add, color: Color(0xFFB8D4D0)),
+              title: const Text(
+                '新建歌单',
+                style: TextStyle(color: Color(0xFFB8D4D0)),
+              ),
+              onTap: () async {
+                Navigator.pop(sheetContext);
+                await _createPlaylistAndAdd(context, repo, songPath);
+              },
+            ),
+            const Divider(height: 1, color: Colors.white12),
+            if (playlists.isEmpty)
+              const Padding(
+                padding: EdgeInsets.symmetric(vertical: 24),
+                child: Text(
+                  '还没有歌单，先新建一个',
+                  style: TextStyle(color: Colors.grey, fontSize: 13),
+                ),
+              )
+            else
+              ConstrainedBox(
+                constraints: const BoxConstraints(maxHeight: 320),
+                child: ListView.builder(
+                  shrinkWrap: true,
+                  itemCount: playlists.length,
+                  itemBuilder: (context, index) {
+                    final playlist = playlists[index];
+                    return ListTile(
+                      leading: const Icon(
+                        Icons.queue_music,
+                        color: Color(0xFFB8D4D0),
+                      ),
+                      title: Text(
+                        playlist.name,
+                        style: const TextStyle(color: Colors.white),
+                      ),
+                      subtitle: Text(
+                        '${playlist.songCount} 首',
+                        style: const TextStyle(
+                            color: Colors.grey, fontSize: 12),
+                      ),
+                      onTap: () async {
+                        final added = await repo.addSongs(
+                          playlist.id,
+                          [songPath],
+                        );
+                        if (!sheetContext.mounted) return;
+                        Navigator.pop(sheetContext);
+                        ScaffoldMessenger.of(context).showSnackBar(
+                          SnackBar(
+                            content: Text(added > 0
+                                ? '已加入「${playlist.name}」'
+                                : '这首歌已经在「${playlist.name}」里了'),
+                          ),
+                        );
+                      },
+                    );
+                  },
+                ),
+              ),
+            const SizedBox(height: 8),
+          ],
+        ),
+      );
+    },
+  );
+}
+
+Future<void> _createPlaylistAndAdd(
+  BuildContext context,
+  PlaylistRepository repo,
+  String songPath,
+) async {
+  final controller = TextEditingController();
+  final name = await showDialog<String>(
+    context: context,
+    builder: (dialogContext) => AlertDialog(
+      backgroundColor: const Color(0xFF1A1F1E),
+      title: const Text('新建歌单', style: TextStyle(color: Colors.white)),
+      content: TextField(
+        controller: controller,
+        autofocus: true,
+        style: const TextStyle(color: Colors.white),
+        decoration: const InputDecoration(hintText: '歌单名称'),
+      ),
+      actions: [
+        TextButton(
+          onPressed: () => Navigator.pop(dialogContext),
+          child: const Text('取消'),
+        ),
+        TextButton(
+          onPressed: () =>
+              Navigator.pop(dialogContext, controller.text.trim()),
+          child: const Text('创建'),
+        ),
+      ],
+    ),
+  );
+
+  if (name == null || name.isEmpty) return;
+
+  try {
+    final playlist = await repo.createPlaylist(name);
+    await repo.addSongs(playlist.id, [songPath]);
+    if (!context.mounted) return;
+    ScaffoldMessenger.of(context).showSnackBar(
+      SnackBar(content: Text('已新建「$name」并加入这首歌')),
+    );
+  } catch (e) {
+    if (!context.mounted) return;
+    ScaffoldMessenger.of(context).showSnackBar(
+      SnackBar(content: Text('新建歌单失败: $e')),
+    );
+  }
+}

test/playlist_summary_test.dart

@@ -0,0 +1,51 @@
+// test/playlist_summary_test.dart
+import 'package:flutter_test/flutter_test.dart';
+import 'package:qingting_player/models/playlist_summary.dart';
+
+void main() {
+  group('PlaylistSummary.fromRow', () {
+    test('完整行解析', () {
+      final s = PlaylistSummary.fromRow({
+        'id': 'abc',
+        'name': '随手',
+        'song_count': 12,
+        'cover_path': '/data/artworks/x.jpg',
+        'updated_at': 1700000000,
+      });
+      expect(s.id, 'abc');
+      expect(s.name, '随手');
+      expect(s.songCount, 12);
+      expect(s.coverPath, '/data/artworks/x.jpg');
+      expect(s.isEmpty, isFalse);
+    });
+
+    test('没有封面时 coverPath 为 null', () {
+      final s = PlaylistSummary.fromRow({
+        'id': 'abc',
+        'name': '车用',
+        'song_count': 3,
+        'cover_path': '',
+      });
+      expect(s.coverPath, isNull);
+      expect(s.songCount, 3);
+    });
+
+    test('空歌单 isEmpty', () {
+      final s = PlaylistSummary.fromRow({
+        'id': 'x',
+        'name': '空',
+        'song_count': 0,
+      });
+      expect(s.isEmpty, isTrue);
+      expect(s.coverPath, isNull);
+    });
+
+    test('字段缺失时不崩', () {
+      final s = PlaylistSummary.fromRow({});
+      expect(s.id, '');
+      expect(s.name, '');
+      expect(s.songCount, 0);
+      expect(s.isEmpty, isTrue);
+    });
+  });
+}