feat: DB-backed favorites replace localStorage

- New clip_favorites table (clip_id, user_id DEFAULT 'default', created_at)
- API: POST /clips/:id/favorite (toggle), GET /favorites, GET /clips/:id/favorite
- Frontend: loads favorites on mount from API, toggles via POST
- Removes localStorage dependency - favorites survive cache clears
- Ready for multi-user: just add real user_id when auth arrives

frontend/src/components/Gallery.tsx

@@ -98,24 +98,32 @@ export default function Gallery({ clips, onPlay, onDelete, onRename, onSelect, o
   const [showFavorites, setShowFavorites] = useState(false)
   const debouncedSearch = useDebounce(search, 300)
   
-  // Favorites
-  const [favorites, setFavorites] = useState<Set<string>>(() => {
-    try {
-      const raw = localStorage.getItem('kithara-favorites')
-      return raw ? new Set(JSON.parse(raw)) : new Set()
-    } catch {
-      return new Set()
-    }
-  })
+  // Favorites (DB-backed)
+  const [favorites, setFavorites] = useState<Set<string>>(new Set())
 
-  const toggleFavorite = useCallback((id: string) => {
-    setFavorites(prev => {
-      const next = new Set(prev)
-      if (next.has(id)) next.delete(id)
-      else next.add(id)
-      localStorage.setItem('kithara-favorites', JSON.stringify([...next]))
-      return next
-    })
+  // Load favorites on mount
+  useEffect(() => {
+    fetch('/api/favorites')
+      .then(res => res.json())
+      .then(data => setFavorites(new Set(data.ids || [])))
+      .catch(() => {}) // silently fail
+  }, [])
+
+  const toggleFavorite = useCallback(async (id: string) => {
+    try {
+      const res = await fetch(`/api/clips/${id}/favorite`, { method: 'POST' })
+      if (res.ok) {
+        const data = await res.json()
+        setFavorites(prev => {
+          const next = new Set(prev)
+          if (data.favorited) next.add(id)
+          else next.delete(id)
+          return next
+        })
+      }
+    } catch {
+      // silently fail
+    }
   }, [])
   
   // Pagination + view