feat: Album artwork generation via Gemini Image models

Backend:
- New image_gen.py module with Gemini image generation client
  - generate_artwork(): generates PNG from text prompt using Google GenAI
  - get_image_gen_config(): loads config from .env (model, API key, safety)
  - get_available_models(): returns list of Gemini image models
  - Safety settings: harassment, hate_speech, sexually_explicit, dangerous
    Configurable via IMAGE_GEN_SAFETY_* env vars (default: BLOCK_ONLY_HIGH)
- New endpoints:
  * POST /api/clips/{id}/generate-artwork — generates and saves artwork
  * GET /api/image-gen/models — lists available models
  * GET /api/image-gen/config — returns safe config (API key masked)
- Default model: gemini-3.1-flash-image-preview
- Fallback API key: LYRIA_API_KEY if IMAGE_GEN_API_KEY not set
- Added google-genai==1.0.0 to requirements.txt

Frontend:
- New ArtworkGenerationModal component for prompt input
  - Textarea with placeholder, loading spinner during generation
  - Styled consistent with SettingsModal (backdrop blur, rounded, dark)
- SettingsModal expanded with Image Generation section (collapsible)
  - Shows current model, API key status, safety filter levels
  - Read-only display — values configured via .env
- DetailPanel: 'Generate' button added next to Upload/Change
  - Passes onGenerateArtwork prop to App.tsx
- App.tsx: handles artwork generation flow, updates state on success

Architecture KB and Tasks KB updated.

backend/requirements.txt

@@ -8,3 +8,4 @@ python-dotenv==1.0.0
 numpy==2.2.0
 librosa==0.10.2
 soundfile==0.12.1
+google-genai==1.0.0