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.

frontend/src/App.tsx

@@ -4,6 +4,7 @@ import Gallery from './components/Gallery'
 import Player from './components/Player'
 import DetailPanel from './components/DetailPanel'
 import SettingsModal from './components/SettingsModal'
+import ArtworkGenerationModal from './components/ArtworkGenerationModal'
 
 interface Clip {
   id: string
@@ -26,6 +27,8 @@ function App() {
   const [isPlaying, setIsPlaying] = useState(false)
   const [selectedClip, setSelectedClip] = useState<Clip | null>(null)
   const [settingsOpen, setSettingsOpen] = useState(false)
+  const [artworkGenOpen, setArtworkGenOpen] = useState(false)
+  const [artworkGenClip, setArtworkGenClip] = useState<Clip | null>(null)
 
   const fetchClips = async () => {
     try {
@@ -134,6 +137,38 @@ function App() {
     }
   }
 
+  const handleGenerateArtwork = async (clip: Clip) => {
+    setArtworkGenClip(clip)
+    setArtworkGenOpen(true)
+  }
+
+  const handleArtworkGenSubmit = async (prompt: string) => {
+    if (!artworkGenClip) return
+    try {
+      const res = await fetch(`/api/clips/${artworkGenClip.id}/generate-artwork`, {
+        method: 'POST',
+        headers: { 'Content-Type': 'application/json' },
+        body: JSON.stringify({ prompt }),
+      })
+      if (res.ok) {
+        const updated = await res.json()
+        setClips(prev => prev.map(c => c.id === updated.id ? updated : c))
+        if (currentClip?.id === updated.id) {
+          setCurrentClip(updated)
+        }
+        if (selectedClip?.id === updated.id) {
+          setSelectedClip(updated)
+        }
+      } else {
+        const err = await res.json()
+        alert(err.detail || 'Failed to generate artwork')
+      }
+    } catch (err) {
+      console.error('Failed to generate artwork:', err)
+      alert('Failed to generate artwork')
+    }
+  }
+
   const handleSelectClip = async (clip: Clip) => {
     setSelectedClip(clip)
     // Fetch analysis if not already cached
@@ -238,6 +273,19 @@ function App() {
           onRemoveArtwork={handleRemoveArtwork}
           onPlay={handlePlay}
           onTogglePlay={() => setIsPlaying(!isPlaying)}
+          onGenerateArtwork={handleGenerateArtwork}
+        />
+      )}
+
+      {artworkGenOpen && artworkGenClip && (
+        <ArtworkGenerationModal
+          isOpen={artworkGenOpen}
+          onClose={() => {
+            setArtworkGenOpen(false)
+            setArtworkGenClip(null)
+          }}
+          onGenerate={handleArtworkGenSubmit}
+          trackName={artworkGenClip.title || artworkGenClip.prompt}
         />
       )}