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/stores/serverStore.ts

@@ -23,6 +23,9 @@ interface ServerStore {
   normalizeAudio: boolean;
   setNormalizeAudio: (value: boolean) => void;
 
+  autoplayOnGenerate: boolean;
+  setAutoplayOnGenerate: (value: boolean) => void;
+
   customModelsDir: string | null;
   setCustomModelsDir: (dir: string | null) => void;
 }
@@ -51,6 +54,9 @@ export const useServerStore = create<ServerStore>()(
       normalizeAudio: true,
       setNormalizeAudio: (value) => set({ normalizeAudio: value }),
 
+      autoplayOnGenerate: true,
+      setAutoplayOnGenerate: (value) => set({ autoplayOnGenerate: value }),
+
       customModelsDir: null,
       setCustomModelsDir: (dir) => set({ customModelsDir: dir }),
     }),