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/app/routes/feed.py

@@ -7,13 +7,24 @@ from sqlalchemy.orm import Session, aliased
 from app import quota
 from app.auth import current_user
 from app.db import get_db
-from app.models import Channel, ChannelTag, Subscription, Tag, User, Video, VideoState
+from app.models import (
+    Channel,
+    ChannelTag,
+    Playlist,
+    PlaylistItem,
+    Subscription,
+    Tag,
+    User,
+    Video,
+    VideoState,
+)
 from app.sync.videos import parse_iso8601_duration
 from app.youtube.client import YouTubeClient, YouTubeError
 
 router = APIRouter(prefix="/api", tags=["feed"])
 
-VALID_STATES = {"new", "watched", "saved", "hidden"}
+# "saved" used to be a status; it's now membership in the built-in Watch later playlist.
+VALID_STATES = {"new", "watched", "hidden"}
 HIDDEN_LIVE = ("live", "upcoming")
 
 # Resume-position thresholds (mirror the client): positions below this are "didn't
@@ -29,6 +40,23 @@ def _channel_url(channel_id: str, handle: str | None) -> str:
     return f"https://www.youtube.com/channel/{channel_id}"
 
 
+def _saved_expr(user: User):
+    """A correlated EXISTS labelled `saved`: is this video in the user's Watch later list?
+    (Watch later is the built-in playlist that replaced the old per-video 'saved' status.)"""
+    return (
+        select(1)
+        .select_from(PlaylistItem)
+        .join(Playlist, Playlist.id == PlaylistItem.playlist_id)
+        .where(
+            Playlist.user_id == user.id,
+            Playlist.kind == "watch_later",
+            PlaylistItem.video_id == Video.id,
+        )
+        .exists()
+        .label("saved")
+    )
+
+
 def _serialize(row) -> dict:
     return {
         "id": row.id,
@@ -45,6 +73,7 @@ def _serialize(row) -> dict:
         "live_status": row.live_status,
         "status": row.status or "new",
         "position_seconds": row.position_seconds or 0,
+        "saved": bool(getattr(row, "saved", False)),
         "watch_url": f"https://www.youtube.com/watch?v={row.id}",
     }
 
@@ -96,6 +125,7 @@ def _filtered_query(
         Video.live_status,
         status_expr.label("status"),
         position_expr.label("position_seconds"),
+        _saved_expr(user),
     ).join(Channel, Channel.id == Video.channel_id)
 
     if scope == "all":
@@ -176,7 +206,7 @@ def _filtered_query(
 
     # Content type: Normal / Shorts / Live·Upcoming as a union of enabled types.
     # Explicit Watched/Saved/Hidden views show every type so nothing goes missing.
-    explicit_view = show in ("watched", "saved", "hidden")
+    explicit_view = show in ("watched", "hidden")
     if not explicit_view:
         type_clauses = []
         if show_normal:
@@ -198,8 +228,6 @@ def _filtered_query(
         )
     elif show == "watched":
         query = query.where(status_expr == "watched")
-    elif show == "saved":
-        query = query.where(status_expr == "saved")
     elif show == "hidden":
         query = query.where(status_expr == "hidden")
     else:  # all