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/utils/format.ts

@@ -21,10 +21,25 @@ export function formatDate(date: string | Date): string {
   } else {
     dateObj = date;
   }
-  
+
   return formatDistance(dateObj, new Date(), { addSuffix: true }).replace(/^about /i, '');
 }
 
+const ENGINE_DISPLAY_NAMES: Record<string, string> = {
+  qwen: 'Qwen',
+  luxtts: 'LuxTTS',
+  chatterbox: 'Chatterbox',
+  chatterbox_turbo: 'Chatterbox Turbo',
+};
+
+export function formatEngineName(engine?: string, modelSize?: string): string {
+  const name = ENGINE_DISPLAY_NAMES[engine ?? 'qwen'] ?? engine ?? 'Qwen';
+  if (engine === 'qwen' && modelSize) {
+    return `${name} ${modelSize}`;
+  }
+  return name;
+}
+
 export function formatFileSize(bytes: number): string {
   if (bytes === 0) return '0 Bytes';
   const k = 1024;