feat: async generation queue with serial execution

Generations now return immediately with a 'generating' status and appear
in history right away. TTS runs in a serial background queue to avoid
GPU contention. Users can kick off multiple generations without blocking.

- Async POST /generate creates DB record immediately, queues TTS work
- Serial generation queue prevents concurrent GPU access (Metal/CUDA/CPU)
- SSE endpoint GET /generate/{id}/status for real-time completion tracking
- Retry endpoint POST /generate/{id}/retry for failed generations
- Store engine and model_size on generation records for retry support
- History cards show animated loader (react-loaders) for generating/playing
- Failed generations show retry button instead of actions menu
- Model downloads happen inline in the queue instead of rejecting with 202
- Stale 'generating' records marked as failed on server startup
- Autoplay on generate setting (default: on)
- Show engine name on generation cards
- Remove sidebar generation spinner
- Checkbox alignment fix in settings

app/src/lib/hooks/useRestoreActiveTasks.tsx

@@ -17,6 +17,7 @@ export function useRestoreActiveTasks() {
   const [activeDownloads, setActiveDownloads] = useState<ActiveDownloadTask[]>([]);
   const setIsGenerating = useGenerationStore((state) => state.setIsGenerating);
   const setActiveGenerationId = useGenerationStore((state) => state.setActiveGenerationId);
+  const addPendingGeneration = useGenerationStore((state) => state.addPendingGeneration);
 
   // Track which downloads we've seen to detect new ones
   const seenDownloadsRef = useRef<Set<string>>(new Set());
@@ -25,10 +26,13 @@ export function useRestoreActiveTasks() {
     try {
       const tasks = await apiClient.getActiveTasks();
 
-      // Update generation state
+      // Update generation state — restore pending generations (e.g., after page refresh)
       if (tasks.generations.length > 0) {
         setIsGenerating(true);
         setActiveGenerationId(tasks.generations[0].task_id);
+        for (const gen of tasks.generations) {
+          addPendingGeneration(gen.task_id);
+        }
       } else {
         // Only clear if we were tracking a generation
         const currentId = useGenerationStore.getState().activeGenerationId;