feat: BPM and musical key detection for all clips

Backend:
- New analysis.py module using librosa for BPM + key detection
  - detect_bpm(): librosa.beat.beat_track() returns tempo
  - detect_key(): Krumhansl-Schmuckler key profiles on chroma features
  - analyze_clip(): combined call returning {bpm, key}
- New dependency: librosa==0.10.2 + soundfile==0.12.1
- Database migrations: bpm (REAL) and key (TEXT) columns added
- New set_analysis() function in database.py
- New endpoint: GET /api/clips/{id}/analysis
  - Returns cached analysis if available
  - Computes on-demand if missing, caches result
  - 400 if clip not ready, 404 if no audio file

Frontend:
- Clip interface extended with bpm?: number | null, key?: string | null
- Gallery: BPM and key badges shown in ribbon and tile views
  - BPM badge: lyria-accent background
  - Key badge: white/5 background
- DetailPanel: BPM and key shown in metadata grid when available
- App.tsx: handleSelectClip() fetches analysis on-demand when opening
  detail panel if not already cached

Docs:
- Architecture KB updated with Audio Analysis section
- Tasks KB: T-028 marked In Progress → will mark Done after commit

frontend/src/components/DetailPanel.tsx

@@ -10,6 +10,8 @@ interface Clip {
   error_message?: string
   file_size_bytes?: number
   artwork_filename?: string | null
+  bpm?: number | null
+  key?: string | null
   created_at: string
 }
 
@@ -258,6 +260,18 @@ export default function DetailPanel({ clip, onClose, onRename, onDelete, onUploa
                 <p className="text-sm text-white">{(clip.file_size_bytes / 1024).toFixed(0)} KB</p>
               </div>
             )}
+            {clip.bpm && (
+              <div>
+                <label className="text-xs text-lyria-muted uppercase tracking-wider block">BPM</label>
+                <p className="text-sm text-white">{Math.round(clip.bpm)}</p>
+              </div>
+            )}
+            {clip.key && (
+              <div>
+                <label className="text-xs text-lyria-muted uppercase tracking-wider block">Key</label>
+                <p className="text-sm text-white">{clip.key}</p>
+              </div>
+            )}
             <div>
               <label className="text-xs text-lyria-muted uppercase tracking-wider block">Created</label>
               <p className="text-sm text-white">{formatDate(clip.created_at)}</p>