feat: Metadata fields (artist, album, comment) with ID3 tag export

Backend:
- Added artist, album, comment columns to clips DB (migration in init_db)
- New update_metadata() function in database.py
- New PATCH /api/clips/{id}/metadata endpoint
- Updated download endpoint to write ID3 tags using mutagen:
  * TIT2 (Title) — from clip title
  * TPE1 (Artist) — from clip artist
  * TALB (Album) — from clip album
  * COMM (Comment) — from clip comment
  * TBPM (BPM) — from detected BPM
  * TKEY (Key) — from detected key
  * TIT1 (Content Group) — prompt for reference
- Added mutagen==1.47.0 to requirements.txt
- Creates temporary copy with tags, original file untouched

Frontend:
- Clip interface updated with artist, album, comment fields
- New MetadataField component in DetailPanel:
  * Inline editing with click-to-edit
  * Input field with lyria-accent border
  * Enter to save, Escape to cancel, blur to save
  * Pencil icon on hover
  * Shows placeholder when empty
- App.tsx: handleUpdateMetadata() function, passed to DetailPanel
- Updates state across clips, currentClip, selectedClip

Architecture KB and Tasks KB updated.

backend/src/main.py

@@ -9,8 +9,8 @@ from dotenv import load_dotenv
 # Load environment variables BEFORE importing lyria_client (it reads env at module level)
 load_dotenv(Path(__file__).parent.parent / ".env")
 
-from models import GenerateRequest, GenerateResponse, ClipList, RenameRequest, 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
+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 lyria_client import generate_clip, save_clip_file
 from waveform import generate_waveform_svg, get_waveform_path
 from analysis import analyze_clip
@@ -120,6 +120,9 @@ async def stream_audio(clip_id: str):
         raise HTTPException(status_code=404, detail="Audio file not found")
     return FileResponse(file_path, media_type="audio/mpeg", filename=clip["filename"])
 
+from mutagen.mp3 import MP3
+from mutagen.id3 import ID3, TIT1, TIT2, TPE1, TALB, COMM, TBPM, TKEY
+
 @app.get("/api/clips/{clip_id}/download")
 async def download_clip(clip_id: str):
     clip = await get_clip(clip_id)
@@ -138,12 +141,66 @@ async def download_clip(clip_id: str):
         safe_name = clip_id
     download_filename = f"{safe_name}.mp3"
     
-    return FileResponse(
-        file_path,
-        media_type="audio/mpeg",
-        filename=download_filename,
-        content_disposition_type="attachment"
-    )
+    # Create a temporary copy with ID3 tags
+    import tempfile
+    import shutil
+    
+    temp_file = tempfile.NamedTemporaryFile(suffix='.mp3', delete=False)
+    temp_path = Path(temp_file.name)
+    temp_file.close()
+    
+    try:
+        # Copy original file
+        shutil.copy2(file_path, temp_path)
+        
+        # Write ID3 tags
+        try:
+            audio = MP3(temp_path)
+            if audio.tags is None:
+                audio.add_tags()
+            
+            # Title
+            if clip.get("title"):
+                audio.tags["TIT2"] = TIT2(encoding=3, text=clip["title"])
+            
+            # Artist
+            if clip.get("artist"):
+                audio.tags["TPE1"] = TPE1(encoding=3, text=clip["artist"])
+            
+            # Album
+            if clip.get("album"):
+                audio.tags["TALB"] = TALB(encoding=3, text=clip["album"])
+            
+            # Comment
+            if clip.get("comment"):
+                audio.tags["COMM"] = COMM(encoding=3, lang='eng', desc='', text=clip["comment"])
+            
+            # BPM
+            if clip.get("bpm"):
+                audio.tags["TBPM"] = TBPM(encoding=3, text=str(int(clip["bpm"])))
+            
+            # Key
+            if clip.get("key"):
+                audio.tags["TKEY"] = TKEY(encoding=3, text=clip["key"])
+            
+            # Also store the prompt in a custom tag for reference
+            if clip.get("prompt"):
+                audio.tags["TIT1"] = TIT1(encoding=3, text=clip["prompt"][:100])
+            
+            audio.save()
+        except Exception as e:
+            print(f"Warning: Failed to write ID3 tags: {e}")
+        
+        return FileResponse(
+            temp_path,
+            media_type="audio/mpeg",
+            filename=download_filename,
+            content_disposition_type="attachment"
+        )
+    except Exception as e:
+        if temp_path.exists():
+            temp_path.unlink()
+        raise HTTPException(status_code=500, detail=f"Failed to prepare download: {str(e)}")
 
 @app.delete("/api/clips/{clip_id}")
 async def remove_clip(clip_id: str):
@@ -377,6 +434,15 @@ async def rename_clip_endpoint(clip_id: str, request: RenameRequest):
     updated = await get_clip(clip_id)
     return updated
 
+@app.patch("/api/clips/{clip_id}/metadata")
+async def update_clip_metadata(clip_id: str, request: MetadataRequest):
+    clip = await get_clip(clip_id)
+    if not clip:
+        raise HTTPException(status_code=404, detail="Clip not found")
+    await update_metadata(clip_id, request.artist, request.album, request.comment)
+    updated = await get_clip(clip_id)
+    return updated
+
 # ─── Admin Endpoints ──────────────────────────────────────────────
 
 @app.get("/api/clips/{clip_id}/analysis")