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/api/client.ts

@@ -200,6 +200,12 @@ class ApiClient {
     });
   }
 
+  async retryGeneration(generationId: string): Promise<GenerationResponse> {
+    return this.request<GenerationResponse>(`/generate/${generationId}/retry`, {
+      method: 'POST',
+    });
+  }
+
   // History
   async listHistory(query?: HistoryQuery): Promise<HistoryListResponse> {
     const params = new URLSearchParams();
@@ -278,6 +284,11 @@ class ApiClient {
     return response.json();
   }
 
+  // Generation status SSE
+  getGenerationStatusUrl(generationId: string): string {
+    return `${this.getBaseUrl()}/generate/${generationId}/status`;
+  }
+
   // Audio
   getAudioUrl(audioId: string): string {
     return `${this.getBaseUrl()}/audio/${audioId}`;