feat: MP3 download/export capability

Backend:
- New endpoint: GET /api/clips/{id}/download
  - Returns MP3 with Content-Disposition: attachment header
  - Filename derived from clip title (sanitized) or prompt, falls back to clip ID
  - Sanitization: alphanumeric + spaces/hyphens/underscores only, max 60 chars
  - Uses FastAPI FileResponse with media_type=audio/mpeg

Frontend:
- Gallery ribbons: Download button (downward arrow icon) next to delete button
  - Uses window.open() to trigger browser download
  - Hover: lyria-accent color
- Gallery tiles: Download button in tile footer action row
  - Small icon (w-3 h-3), same hover behavior
- DetailPanel: Full-width 'Download MP3' button above Delete Track
  - lyria-accent styled, with download icon
  - Uses <a href> with download attribute for native browser handling

Architecture KB updated with Export/Download section.

frontend/src/components/DetailPanel.tsx

@@ -302,7 +302,17 @@ export default function DetailPanel({ clip, onClose, onRename, onDelete, onUploa
           </div>
 
           {/* Actions */}
-          <div className="pt-3 border-t border-white/5">
+          <div className="pt-3 border-t border-white/5 space-y-2">
+            <a
+              href={`/api/clips/${clip.id}/download`}
+              download
+              className="w-full py-2 text-sm text-lyria-accent bg-lyria-accent/10 hover:bg-lyria-accent/20 rounded-md transition-colors flex items-center justify-center gap-2"
+            >
+              <svg className="w-4 h-4" fill="none" stroke="currentColor" viewBox="0 0 24 24">
+                <path strokeLinecap="round" strokeLinejoin="round" strokeWidth={2} d="M4 16v1a3 3 0 003 3h10a3 3 0 003-3v-1m-4-4l-4 4m0 0l-4-4m4 4V4" />
+              </svg>
+              Download MP3
+            </a>
             <button
               onClick={() => { onDelete(clip.id); onClose() }}
               className="w-full py-2 text-sm text-red-400 bg-red-500/10 hover:bg-red-500/20 rounded-md transition-colors"