feat(playlists): unify Saved into a built-in Watch later playlist

The old per-video status='saved' becomes membership in a built-in, undeletable
'watch_later' playlist. Migration 0012 moves existing saved videos into each user's
Watch later list and demotes the states to 'new'. The feed/playlist serializers now
expose a 'saved' boolean (EXISTS in watch_later); the card bookmark toggles watch_later
via new /api/playlists/watch-later add/remove endpoints (create-on-demand) instead of
setting a status. Removed the 'saved' show-filter and status. Watch later shows a
localized name and hides rename/delete in the Playlists page and add-to-playlist popover.

backend/alembic/versions/0012_watch_later.py

@@ -0,0 +1,65 @@
+"""unify Saved into a built-in Watch later playlist
+
+Revision ID: 0012_watch_later
+Revises: 0011_playlists
+Create Date: 2026-06-15
+
+Migrates the old per-video status='saved' into membership of a built-in 'watch_later'
+playlist (one per user who had saved videos), then demotes those states to 'new'.
+"""
+from typing import Sequence, Union
+
+from alembic import op
+
+revision: str = "0012_watch_later"
+down_revision: Union[str, None] = "0011_playlists"
+branch_labels: Union[str, Sequence[str], None] = None
+depends_on: Union[str, Sequence[str], None] = None
+
+
+def upgrade() -> None:
+    # 1) A Watch later playlist for each user that has saved videos (and none yet).
+    op.execute(
+        """
+        INSERT INTO playlists (user_id, name, kind, source, dirty, position, created_at, updated_at)
+        SELECT DISTINCT vs.user_id, 'Watch later', 'watch_later', 'local', false, 0, now(), now()
+        FROM video_states vs
+        WHERE vs.status = 'saved'
+          AND NOT EXISTS (
+            SELECT 1 FROM playlists p
+            WHERE p.user_id = vs.user_id AND p.kind = 'watch_later'
+          )
+        """
+    )
+    # 2) Add each saved video as an item, ordered (newest-saved first → position 0).
+    op.execute(
+        """
+        INSERT INTO playlist_items (playlist_id, video_id, position, added_at)
+        SELECT p.id, vs.video_id,
+               row_number() OVER (
+                 PARTITION BY vs.user_id
+                 ORDER BY COALESCE(vs.watched_at, vs.updated_at) DESC
+               ) - 1,
+               COALESCE(vs.watched_at, vs.updated_at, now())
+        FROM video_states vs
+        JOIN playlists p ON p.user_id = vs.user_id AND p.kind = 'watch_later'
+        WHERE vs.status = 'saved'
+        ON CONFLICT (playlist_id, video_id) DO NOTHING
+        """
+    )
+    # 3) Demote the migrated saved states to the default.
+    op.execute("UPDATE video_states SET status = 'new' WHERE status = 'saved'")
+
+
+def downgrade() -> None:
+    # Best-effort reverse: restore status='saved' for videos in watch_later lists, then drop them.
+    op.execute(
+        """
+        UPDATE video_states vs SET status = 'saved'
+        FROM playlist_items pi
+        JOIN playlists p ON p.id = pi.playlist_id AND p.kind = 'watch_later'
+        WHERE pi.video_id = vs.video_id AND p.user_id = vs.user_id
+          AND vs.status = 'new'
+        """
+    )
+    op.execute("DELETE FROM playlists WHERE kind = 'watch_later'")