refactor(playlists,feed): share feed-row projection & playlist helpers

- feed_columns() is the single feed-row projection, consumed by both /feed and the
  playlist-detail query (was copy-pasted, drift risk on every new column).
- _replace_items_from_youtube() dedups the YouTube playlist-mirror block shared by
  sync_user_playlists and repull_playlist.
- _next_playlist_position/_next_item_position/_add_item replace the append-position
  idiom (x4) and the exists-then-insert blocks (x2).

backend/app/routes/feed.py

@@ -62,6 +62,29 @@ def _saved_expr(user: User):
     )
 
 
+def feed_columns(user: User, state) -> list:
+    """The shared feed-row projection (joined Channel fields + per-user watch state +
+    `saved`), consumed by both the feed query and the playlist-detail query so a new column
+    is added in exactly one place. `state` is the caller's aliased VideoState (outer-joined)."""
+    return [
+        Video.id,
+        Video.title,
+        Video.channel_id,
+        Channel.title.label("channel_title"),
+        Channel.thumbnail_url.label("channel_thumbnail"),
+        Channel.handle.label("channel_handle"),
+        Video.published_at,
+        Video.thumbnail_url,
+        Video.duration_seconds,
+        Video.view_count,
+        Video.is_short,
+        Video.live_status,
+        func.coalesce(state.status, "new").label("status"),
+        func.coalesce(state.position_seconds, 0).label("position_seconds"),
+        _saved_expr(user),
+    ]
+
+
 def _serialize(row) -> dict:
     return {
         "id": row.id,
@@ -115,23 +138,9 @@ def _filtered_query(
     status_expr = func.coalesce(state.status, "new")
     position_expr = func.coalesce(state.position_seconds, 0)
 
-    query = select(
-        Video.id,
-        Video.title,
-        Video.channel_id,
-        Channel.title.label("channel_title"),
-        Channel.thumbnail_url.label("channel_thumbnail"),
-        Channel.handle.label("channel_handle"),
-        Video.published_at,
-        Video.thumbnail_url,
-        Video.duration_seconds,
-        Video.view_count,
-        Video.is_short,
-        Video.live_status,
-        status_expr.label("status"),
-        position_expr.label("position_seconds"),
-        _saved_expr(user),
-    ).join(Channel, Channel.id == Video.channel_id)
+    query = select(*feed_columns(user, state)).join(
+        Channel, Channel.id == Video.channel_id
+    )
 
     if scope == "all":
         # Whole shared catalog; subscription is optional (only for priority sort).