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/requirements.txt

@@ -9,3 +9,4 @@ numpy==2.2.0
 librosa==0.10.2
 soundfile==0.12.1
 google-genai==1.0.0
+mutagen==1.47.0