feat(playlists): reset a playlist to its YouTube state (discard local edits)

The in-session undo/redo is lost when switching playlists, leaving no way to undo
local edits to a YouTube-linked list afterwards. Add a per-playlist 'Reset to YouTube'
action (shown when a linked playlist is dirty) that force-repulls that single playlist
from YouTube, replacing its items/order/name and clearing dirty — even though the bulk
read-sync skips dirty playlists. Backend: repull_playlist() + POST /api/playlists/
{id}/revert-youtube (read-scope gated). Confirm dialog (destructive). Trilingual.

backend/app/sync/playlists.py

@@ -305,6 +305,39 @@ def push_playlist(db: Session, user: User, pl: Playlist) -> dict:
     }
 
 
+def repull_playlist(db: Session, user: User, pl: Playlist) -> dict:
+    """Force-refresh ONE YouTube-linked playlist from YouTube, discarding local edits and
+    clearing dirty. This is the per-playlist 'reset to YouTube' — unlike the bulk read-sync
+    it deliberately does NOT skip a dirty playlist (overwriting local changes is the point).
+    Works for both mirrors (source='youtube') and exported local playlists (yt_playlist_id)."""
+    if not pl.yt_playlist_id:
+        raise YouTubeError("Playlist is not linked to YouTube")
+    with YouTubeClient(db, user) as yt:
+        snippet = yt.get_playlist_snippet(pl.yt_playlist_id)
+        if snippet is None:
+            raise YouTubeError("Playlist no longer exists on YouTube")
+        ids = list(yt.iter_my_playlist_video_ids(pl.yt_playlist_id))
+        _ensure_videos(db, yt, ids)
+        present = (
+            set(db.execute(select(Video.id).where(Video.id.in_(ids))).scalars().all())
+            if ids
+            else set()
+        )
+        ordered: list[str] = []
+        seen: set[str] = set()
+        for v in ids:
+            if v in present and v not in seen:
+                seen.add(v)
+                ordered.append(v)
+        pl.name = snippet.get("title") or pl.name
+        db.execute(delete(PlaylistItem).where(PlaylistItem.playlist_id == pl.id))
+        for pos, vid in enumerate(ordered):
+            db.add(PlaylistItem(playlist_id=pl.id, video_id=vid, position=pos))
+        pl.dirty = False
+        db.commit()
+    return {"items": len(ordered)}
+
+
 def sync_all_playlists(db: Session) -> dict:
     """Scheduler entry point: mirror playlists for every user with a read scope + token."""
     users = (