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/routes/sync.py

@@ -1,5 +1,5 @@
 from fastapi import APIRouter, Depends, HTTPException
-from sqlalchemy import func, select
+from sqlalchemy import and_, case, func, select
 from sqlalchemy.orm import Session
 
 from app import quota, state
@@ -93,6 +93,50 @@ def sync_status(
     }
 
 
+@router.get("/my-status")
+def my_status(
+    user: User = Depends(current_user), db: Session = Depends(get_db)
+) -> dict:
+    """Per-user sync progress: how far the channels *this* user subscribes to have synced."""
+    sub_join = and_(
+        Subscription.channel_id == Channel.id, Subscription.user_id == user.id
+    )
+    total, details_synced, recent_synced, deep_done = db.execute(
+        select(
+            func.count(Channel.id),
+            func.count(Channel.details_synced_at),
+            func.count(Channel.recent_synced_at),
+            func.sum(case((Channel.backfill_done.is_(True), 1), else_=0)),
+        )
+        .select_from(Channel)
+        .join(Subscription, sub_join)
+    ).one()
+    total = total or 0
+    deep_done = int(deep_done or 0)
+    my_videos = db.scalar(
+        select(func.count(Video.id)).join(
+            Subscription,
+            and_(
+                Subscription.channel_id == Video.channel_id,
+                Subscription.user_id == user.id,
+                Subscription.hidden.is_(False),
+            ),
+        )
+    )
+    return {
+        "channels_total": total,
+        "channels_details_synced": details_synced,
+        "channels_recent_synced": recent_synced,
+        "channels_deep_done": deep_done,
+        "channels_recent_pending": total - recent_synced,
+        "channels_deep_pending": total - deep_done,
+        "my_videos": my_videos or 0,
+        "quota_used_today": quota.units_used_today(db),
+        "quota_remaining_today": quota.remaining_today(db),
+        "paused": state.is_sync_paused(db),
+    }
+
+
 @router.post("/pause")
 def pause_sync(
     user: User = Depends(current_user), db: Session = Depends(get_db)