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/hooks/useGenerationProgress.ts

@@ -0,0 +1,118 @@
+import { useQueryClient } from '@tanstack/react-query';
+import { useEffect, useRef } from 'react';
+import { useToast } from '@/components/ui/use-toast';
+import { apiClient } from '@/lib/api/client';
+import { useGenerationStore } from '@/stores/generationStore';
+import { usePlayerStore } from '@/stores/playerStore';
+import { useServerStore } from '@/stores/serverStore';
+
+interface GenerationStatusEvent {
+  id: string;
+  status: 'generating' | 'completed' | 'failed';
+  duration?: number;
+  error?: string;
+}
+
+/**
+ * Subscribes to SSE for all pending generations. When a generation completes,
+ * invalidates the history query, removes it from pending, and auto-plays
+ * if the player is idle.
+ */
+export function useGenerationProgress() {
+  const queryClient = useQueryClient();
+  const { toast } = useToast();
+  const pendingIds = useGenerationStore((s) => s.pendingGenerationIds);
+  const removePendingGeneration = useGenerationStore((s) => s.removePendingGeneration);
+  const isPlaying = usePlayerStore((s) => s.isPlaying);
+  const setAudioWithAutoPlay = usePlayerStore((s) => s.setAudioWithAutoPlay);
+  const autoplayOnGenerate = useServerStore((s) => s.autoplayOnGenerate);
+
+  // Keep refs to avoid stale closures in EventSource handlers
+  const isPlayingRef = useRef(isPlaying);
+  const autoplayRef = useRef(autoplayOnGenerate);
+  isPlayingRef.current = isPlaying;
+  autoplayRef.current = autoplayOnGenerate;
+
+  // Track active EventSource instances
+  const eventSourcesRef = useRef<Map<string, EventSource>>(new Map());
+
+  useEffect(() => {
+    const currentSources = eventSourcesRef.current;
+
+    // Close SSE connections for IDs no longer pending
+    for (const [id, source] of currentSources.entries()) {
+      if (!pendingIds.has(id)) {
+        source.close();
+        currentSources.delete(id);
+      }
+    }
+
+    // Open SSE connections for new pending IDs
+    for (const id of pendingIds) {
+      if (currentSources.has(id)) continue;
+
+      const url = apiClient.getGenerationStatusUrl(id);
+      const source = new EventSource(url);
+
+      source.onmessage = (event) => {
+        try {
+          const data: GenerationStatusEvent = JSON.parse(event.data);
+
+          if (data.status === 'completed') {
+            source.close();
+            currentSources.delete(id);
+            removePendingGeneration(id);
+
+            // Refresh history to pick up the completed generation
+            queryClient.invalidateQueries({ queryKey: ['history'] });
+
+            toast({
+              title: 'Generation complete!',
+              description: data.duration
+                ? `Audio generated (${data.duration.toFixed(2)}s)`
+                : 'Audio generated',
+            });
+
+            // Auto-play if enabled and nothing is currently playing
+            if (autoplayRef.current && !isPlayingRef.current) {
+              const genAudioUrl = apiClient.getAudioUrl(id);
+              setAudioWithAutoPlay(genAudioUrl, id, '', '');
+            }
+          } else if (data.status === 'failed') {
+            source.close();
+            currentSources.delete(id);
+            removePendingGeneration(id);
+
+            queryClient.invalidateQueries({ queryKey: ['history'] });
+
+            toast({
+              title: 'Generation failed',
+              description: data.error || 'An error occurred during generation',
+              variant: 'destructive',
+            });
+          }
+        } catch {
+          // Ignore parse errors from heartbeats etc
+        }
+      };
+
+      source.onerror = () => {
+        // EventSource auto-reconnects, but if we get repeated errors
+        // just clean up
+        source.close();
+        currentSources.delete(id);
+        removePendingGeneration(id);
+      };
+
+      currentSources.set(id, source);
+    }
+
+    return () => {
+      // Cleanup on unmount
+      for (const source of currentSources.values()) {
+        source.close();
+      }
+      currentSources.clear();
+    };
+  }, [pendingIds, removePendingGeneration, queryClient, toast, setAudioWithAutoPlay]);
+}