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/index.css

@@ -1,4 +1,5 @@
 @import "tailwindcss" source(".");
+@import "loaders.css/loaders.min.css";
 
 @theme {
   --radius-sm: calc(var(--radius) - 4px);
@@ -155,3 +156,18 @@
   animation: fadeIn 0.5s ease-out 0.15s forwards;
   opacity: 0;
 }
+
+/* react-loaders */
+.line-scale-pulse-out-rapid > div,
+.line-scale > div {
+  background-color: hsl(var(--accent)) !important;
+}
+
+.loader-hidden {
+  display: block;
+}
+
+.loader-hidden > div > div {
+  animation-play-state: paused !important;
+  background-color: hsl(var(--muted-foreground)) !important;
+}