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"

frontend/src/components/Gallery.tsx

@@ -303,6 +303,18 @@ export default function Gallery({ clips, onPlay, onDelete, onRename, onSelect, o
                   <path strokeLinecap="round" strokeLinejoin="round" strokeWidth={2} d="M4 16l4.586-4.586a2 2 0 012.828 0L16 16m-2-2l1.586-1.586a2 2 0 012.828 0L20 14m-6-6h.01M6 20h12a2 2 0 002-2V6a2 2 0 00-2-2H6a2 2 0 00-2 2v12a2 2 0 002 2z" />
                 </svg>
               </button>
+              <button
+                onClick={(e) => {
+                  e.stopPropagation()
+                  window.open(`/api/clips/${clip.id}/download`, '_blank')
+                }}
+                className="text-lyria-muted hover:text-lyria-accent transition-colors shrink-0"
+                title="Download MP3"
+              >
+                <svg className="w-4 h-4" fill="none" stroke="currentColor" viewBox="0 0 24 24">
+                  <path strokeLinecap="round" strokeLinejoin="round" strokeWidth={2} d="M4 16v1a3 3 0 003 3h10a3 3 0 003-3v-1m-4-4l-4 4m0 0l-4-4m4 4V4" />
+                </svg>
+              </button>
               <button
                 onClick={(e) => { e.stopPropagation(); onDelete(clip.id) }}
                 className="text-lyria-muted hover:text-red-400 transition-colors shrink-0"
@@ -449,6 +461,18 @@ export default function Gallery({ clips, onPlay, onDelete, onRename, onSelect, o
                     <path strokeLinecap="round" strokeLinejoin="round" strokeWidth={2} d="M11 5H6a2 2 0 00-2 2v11a2 2 0 002 2h11a2 2 0 002-2v-5m-1.414-9.414a2 2 0 112.828 2.828L11.828 15H9v-2.828l8.586-8.586z" />
                   </svg>
                 </button>
+                <button
+                  onClick={(e) => {
+                    e.stopPropagation()
+                    window.open(`/api/clips/${clip.id}/download`, '_blank')
+                  }}
+                  className="text-lyria-muted/40 hover:text-lyria-accent transition-colors text-xs"
+                  title="Download MP3"
+                >
+                  <svg className="w-3 h-3" 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>
+                </button>
                 <button
                   onClick={(e) => { e.stopPropagation(); onDelete(clip.id) }}
                   className="text-lyria-muted/40 hover:text-red-400 transition-colors text-xs"