feat: M4 (part 1) — feed API, watch state, user preferences

- GET /api/feed: filter by tags (and/or), channel, duration, age, search; sort
  newest/oldest/views/duration/shuffle; default hides Shorts, live/upcoming and
  watched (was_live VODs stay visible); offset paging with has_more
- POST /api/videos/{id}/state for watched/saved/hidden/new
- User.preferences JSON + GET /api/me and PUT /api/me/preferences
- Migration 0004 (preferences column + video_states table)

backend/app/main.py

@@ -9,7 +9,7 @@ from starlette.middleware.sessions import SessionMiddleware
 
 from app import auth
 from app.config import settings
-from app.routes import health, sync, tags
+from app.routes import feed, health, me, sync, tags
 from app.scheduler import shutdown_scheduler, start_scheduler
 
 
@@ -44,6 +44,8 @@ app.include_router(health.router)
 app.include_router(auth.router)
 app.include_router(sync.router)
 app.include_router(tags.router)
+app.include_router(feed.router)
+app.include_router(me.router)
 
 STATIC_DIR = Path(__file__).parent / "static"
 app.mount("/assets", StaticFiles(directory=STATIC_DIR / "assets"), name="assets")