feat(m5a): channel manager, tabbed settings panel, per-user sync status

Backend: /api/channels (list + PATCH priority/hidden + attach/detach user tags),
user-tag CRUD on /api/tags, /api/sync/my-status (per-user channel sync counts).
Frontend: feed|channels page nav (URL-synced) from the account menu; a slide-in
tabbed Settings panel (Appearance, Notifications=6b sound+duration, Sync status +
actions + admin pause/resume, Account); a channel manager with priority, hide,
per-channel user tags, sync badges and 'view in feed'. Notifications now honor the
configurable sound + auto-dismiss settings.

backend/app/main.py

@@ -26,7 +26,7 @@ from starlette.middleware.sessions import SessionMiddleware
 
 from app import auth
 from app.config import settings
-from app.routes import feed, health, me, sync, tags
+from app.routes import channels, feed, health, me, sync, tags
 from app.scheduler import shutdown_scheduler, start_scheduler
 
 
@@ -65,6 +65,7 @@ app.include_router(sync.router)
 app.include_router(tags.router)
 app.include_router(feed.router)
 app.include_router(me.router)
+app.include_router(channels.router)
 
 # The built SPA (populated by the Docker frontend build stage).
 STATIC_DIR = Path(__file__).parent / "static_spa"