perf(database): add composite indexes for hot ORDER BY paths

The existing single-column indexes force a sort step for the most common
list queries. These three composite indexes let SQLite scan in index
order and skip the sort entirely:

- recently_viewed(playlist_id, viewed_at DESC):
  getRecentItems(): WHERE playlist_id = ? ORDER BY viewed_at DESC LIMIT 100
- favorites(playlist_id, position, added_at DESC):
  getFavorites/getGlobalFavorites/getAllGlobalFavorites():
  WHERE playlist_id = ? ORDER BY position ASC, added_at DESC
- playback_positions(playlist_id, updated_at DESC):
  getRecentPlaybackPositions(): WHERE playlist_id = ? ORDER BY updated_at DESC LIMIT 20

CREATE INDEX IF NOT EXISTS so existing DBs pick up the new indexes on
next startup with no migration required. Mirrored in the Drizzle schema
using sql\`col DESC\` so the ORM stays the source of truth.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
Entire-Checkpoint: 3d6901072049

libs/shared/database/src/lib/schema.ts

@@ -129,6 +129,10 @@ export const recentlyViewed = sqliteTable(
         ).on(table.contentId, table.playlistId),
         playlistIdx: index('recently_viewed_playlist_idx').on(table.playlistId),
         viewedAtIdx: index('recently_viewed_viewed_at_idx').on(table.viewedAt),
+        playlistViewedIdx: index('recently_viewed_playlist_viewed_idx').on(
+            table.playlistId,
+            sql`${table.viewedAt} DESC`
+        ),
     })
 );
 
@@ -153,6 +157,11 @@ export const favorites = sqliteTable(
         ).on(table.contentId, table.playlistId),
         playlistIdx: index('favorites_playlist_idx').on(table.playlistId),
         contentIdx: index('favorites_content_idx').on(table.contentId),
+        playlistPositionIdx: index('favorites_playlist_position_idx').on(
+            table.playlistId,
+            table.position,
+            sql`${table.addedAt} DESC`
+        ),
     })
 );
 
@@ -241,6 +250,9 @@ export const playbackPositions = sqliteTable(
             table.seriesXtreamId
         ),
         updatedIdx: index('playback_positions_updated_idx').on(table.updatedAt),
+        playlistUpdatedIdx: index(
+            'playback_positions_playlist_updated_idx'
+        ).on(table.playlistId, sql`${table.updatedAt} DESC`),
     })
 );