Add playlists support and fix explore older videos

- New playlists router: fetch channel playlists from YouTube, index
  playlist videos, browse by playlist with pagination
- Playlist model gets video_ids column to store ordered video list
- Register playlists router in main.py with DB migration
- Add Playlists tab to Channel page: grid of playlist cards, click to
  browse videos, index/re-index per playlist
- Fix explore older videos skipping all entries without published_at;
  flat-playlist entries for older videos rarely include timestamp data

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

backend/main.py

@@ -6,7 +6,7 @@ from fastapi.staticfiles import StaticFiles
 from .config import settings
 from .database import init_db, SessionLocal
 from .services import ytdlp as ytdlp_service
-from .routers import auth, channels, videos, search, downloads, discovery, settings as settings_router, stats as stats_router, export as export_router, collections as collections_router, admin as admin_router
+from .routers import auth, channels, videos, search, downloads, discovery, settings as settings_router, stats as stats_router, export as export_router, collections as collections_router, admin as admin_router, playlists as playlists_router
 
 app = FastAPI(title="YouTube Hub", version="0.1.0")
 
@@ -29,6 +29,7 @@ app.include_router(stats_router.router, prefix="/api/stats", tags=["stats"])
 app.include_router(export_router.router, prefix="/api/export", tags=["export"])
 app.include_router(collections_router.router, prefix="/api/collections", tags=["collections"])
 app.include_router(admin_router.router, prefix="/api/admin", tags=["admin"])
+app.include_router(playlists_router.router, prefix="/api/playlists", tags=["playlists"])
 
 
 os.makedirs(settings.download_path, exist_ok=True)
@@ -73,6 +74,19 @@ def on_startup():
         "ALTER TABLE user_settings ADD COLUMN use_oauth2 INTEGER DEFAULT 0",
         "ALTER TABLE user_settings ADD COLUMN sync_interval_hours INTEGER DEFAULT 0",
         "ALTER TABLE user_settings ADD COLUMN subtitle_langs TEXT DEFAULT ''",
+        """CREATE TABLE IF NOT EXISTS playlists (
+            id INTEGER PRIMARY KEY AUTOINCREMENT,
+            youtube_playlist_id TEXT NOT NULL UNIQUE,
+            channel_id INTEGER REFERENCES channels(id) ON DELETE CASCADE,
+            title TEXT NOT NULL,
+            description TEXT,
+            thumbnail_url TEXT,
+            video_count INTEGER DEFAULT 0,
+            video_ids TEXT,
+            indexed_at DATETIME,
+            crawled_at DATETIME DEFAULT CURRENT_TIMESTAMP
+        )""",
+        "ALTER TABLE playlists ADD COLUMN video_ids TEXT",
         """CREATE TABLE IF NOT EXISTS search_history (
             id INTEGER PRIMARY KEY AUTOINCREMENT,
             user_id INTEGER NOT NULL REFERENCES users(id) ON DELETE CASCADE,