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/components/ServerSettings/ConnectionForm.tsx

@@ -124,6 +124,7 @@ export function ConnectionForm() {
           <div className="flex items-start space-x-3">
             <Checkbox
               id="keepServerRunning"
+              className="mt-[6px]"
               checked={keepServerRunningOnClose}
               onCheckedChange={(checked: boolean) => {
                 setKeepServerRunningOnClose(checked);
@@ -158,6 +159,7 @@ export function ConnectionForm() {
             <div className="flex items-start space-x-3">
               <Checkbox
                 id="allowNetworkAccess"
+                className="mt-[6px]"
                 checked={mode === 'remote'}
                 onCheckedChange={(checked: boolean) => {
                   setMode(checked ? 'remote' : 'local');