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.

frontend/src/App.tsx

@@ -19,6 +19,9 @@ interface Clip {
   bpm?: number | null
   key?: string | null
   prompt_structure?: Record<string, string> | null
+  artist?: string | null
+  album?: string | null
+  comment?: string | null
   created_at: string
 }
 
@@ -187,6 +190,32 @@ function App() {
     }
   }
 
+  const handleUpdateMetadata = async (id: string, artist: string, album: string, comment: string) => {
+    try {
+      const res = await fetch(`/api/clips/${id}/metadata`, {
+        method: 'PATCH',
+        headers: { 'Content-Type': 'application/json' },
+        body: JSON.stringify({ artist, album, comment }),
+      })
+      if (res.ok) {
+        const updated = await res.json()
+        setClips(prev => prev.map(c => c.id === updated.id ? updated : c))
+        if (currentClip?.id === updated.id) {
+          setCurrentClip(updated)
+        }
+        if (selectedClip?.id === updated.id) {
+          setSelectedClip(updated)
+        }
+      } else {
+        const err = await res.json()
+        alert(err.detail || 'Failed to update metadata')
+      }
+    } catch (err) {
+      console.error('Failed to update metadata:', err)
+      alert('Failed to update metadata')
+    }
+  }
+
   const handleClearWaveforms = async () => {
     try {
       await fetch('/api/admin/clear-waveforms', { method: 'POST' })
@@ -275,6 +304,7 @@ function App() {
           onPlay={handlePlay}
           onTogglePlay={() => setIsPlaying(!isPlaying)}
           onGenerateArtwork={handleGenerateArtwork}
+          onUpdateMetadata={handleUpdateMetadata}
         />
       )}