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

backend/src/database.py

@@ -50,6 +50,15 @@ CREATE TABLE IF NOT EXISTS clips (
 -- ALTER TABLE clips ADD COLUMN title TEXT; -- run separately if needed
 
 CREATE INDEX IF NOT EXISTS idx_clips_created ON clips(created_at DESC);
+-- clip_favorites: many-to-many join for user favorites (user_id defaults to 'default' until auth)
+CREATE TABLE IF NOT EXISTS clip_favorites (
+    clip_id TEXT NOT NULL REFERENCES clips(id) ON DELETE CASCADE,
+    user_id TEXT NOT NULL DEFAULT 'default',
+    created_at TIMESTAMP DEFAULT CURRENT_TIMESTAMP,
+    PRIMARY KEY (clip_id, user_id)
+);
+
+CREATE INDEX IF NOT EXISTS idx_favorites_user ON clip_favorites(user_id, created_at DESC);
 
 -- Migration: add artwork_filename column to existing databases
 -- ALTER TABLE clips ADD COLUMN artwork_filename TEXT; -- run separately if needed
@@ -221,4 +230,49 @@ async def delete_clip(clip_id: str):
 async def delete_all_clips():
     async with aiosqlite.connect(DB_PATH) as db:
         await db.execute("DELETE FROM clips")
-        await db.commit()
+        await db.commit()
+
+# ─── Favorites ───────────────────────────────────────────────────────────────
+
+async def toggle_favorite(clip_id: str, user_id: str = "default") -> bool:
+    """Toggle favorite status. Returns True if now favorited, False if removed."""
+    async with aiosqlite.connect(DB_PATH) as db:
+        # Check if already favorited
+        async with db.execute(
+            "SELECT 1 FROM clip_favorites WHERE clip_id = ? AND user_id = ?",
+            (clip_id, user_id)
+        ) as cursor:
+            exists = await cursor.fetchone()
+        if exists:
+            await db.execute(
+                "DELETE FROM clip_favorites WHERE clip_id = ? AND user_id = ?",
+                (clip_id, user_id)
+            )
+            await db.commit()
+            return False
+        else:
+            await db.execute(
+                "INSERT INTO clip_favorites (clip_id, user_id) VALUES (?, ?)",
+                (clip_id, user_id)
+            )
+            await db.commit()
+            return True
+
+async def get_favorites(user_id: str = "default") -> list[str]:
+    """Get list of favorited clip IDs for a user."""
+    async with aiosqlite.connect(DB_PATH) as db:
+        async with db.execute(
+            "SELECT clip_id FROM clip_favorites WHERE user_id = ? ORDER BY created_at DESC",
+            (user_id,)
+        ) as cursor:
+            rows = await cursor.fetchall()
+            return [row[0] for row in rows]
+
+async def is_favorite(clip_id: str, user_id: str = "default") -> bool:
+    """Check if a clip is favorited by a user."""
+    async with aiosqlite.connect(DB_PATH) as db:
+        async with db.execute(
+            "SELECT 1 FROM clip_favorites WHERE clip_id = ? AND user_id = ?",
+            (clip_id, user_id)
+        ) as cursor:
+            return await cursor.fetchone() is not None

backend/src/main.py

@@ -10,7 +10,7 @@ from dotenv import load_dotenv
 load_dotenv(Path(__file__).parent.parent / ".env")
 
 from models import GenerateRequest, GenerateResponse, ClipList, RenameRequest, MetadataRequest, GenerateArtworkRequest, LYRIA_MODELS, DEFAULT_MODEL
-from database import init_db, create_clip, get_clips, get_clip, delete_clip, update_clip_status, rename_clip, set_artwork, remove_artwork, delete_all_clips, set_analysis, update_metadata
+from database import init_db, create_clip, get_clips, get_clip, delete_clip, update_clip_status, rename_clip, set_artwork, remove_artwork, delete_all_clips, set_analysis, update_metadata, toggle_favorite, get_favorites, is_favorite
 from kithara_client import generate_clip, save_clip_file
 from waveform import generate_waveform_svg, get_waveform_path
 from analysis import analyze_clip
@@ -569,6 +569,35 @@ async def delete_all_clips_endpoint():
     await delete_all_clips()
     return {"deleted": len(all_clips)}
 
+# ─── Favorites ─────────────────────────────────────────────────────────────────
+
+@app.post("/api/clips/{clip_id}/favorite")
+async def toggle_favorite_endpoint(clip_id: str):
+    """Toggle favorite status for a clip. Returns {favorited: bool}."""
+    clip = await get_clip(clip_id)
+    if not clip:
+        raise HTTPException(status_code=404, detail="Clip not found")
+    favorited = await toggle_favorite(clip_id)
+    return {"favorited": favorited}
+
+
+@app.get("/api/favorites")
+async def get_favorites_endpoint():
+    """Get list of favorited clip IDs for the current user."""
+    ids = await get_favorites()
+    return {"ids": ids}
+
+
+@app.get("/api/clips/{clip_id}/favorite")
+async def check_favorite_endpoint(clip_id: str):
+    """Check if a clip is favorited."""
+    clip = await get_clip(clip_id)
+    if not clip:
+        raise HTTPException(status_code=404, detail="Clip not found")
+    is_fav = await is_favorite(clip_id)
+    return {"favorited": is_fav}
+
+
 if __name__ == "__main__":
     import uvicorn
     uvicorn.run(app, host="0.0.0.0", port=8000)