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

backend/requirements.txt

@@ -6,3 +6,5 @@ pydantic>=2.10.0
 python-multipart==0.0.12
 python-dotenv==1.0.0
 numpy==2.2.0
+librosa==0.10.2
+soundfile==0.12.1