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

backend/database.py

@@ -45,10 +45,14 @@ class Generation(Base):
     profile_id = Column(String, ForeignKey("profiles.id"), nullable=False)
     text = Column(Text, nullable=False)
     language = Column(String, default="en")
-    audio_path = Column(String, nullable=False)
-    duration = Column(Float, nullable=False)
+    audio_path = Column(String, nullable=True)
+    duration = Column(Float, nullable=True)
     seed = Column(Integer)
     instruct = Column(Text)
+    engine = Column(String, default="qwen")
+    model_size = Column(String, nullable=True)
+    status = Column(String, default="completed")  # generating, completed, failed
+    error = Column(Text, nullable=True)
     created_at = Column(DateTime, default=datetime.utcnow)
 
 
@@ -288,6 +292,36 @@ def _run_migrations(engine):
                 conn.commit()
                 print("Added avatar_path column to profiles")
 
+    # Migration: Add status and error columns to generations table
+    if 'generations' in inspector.get_table_names():
+        columns = {col['name'] for col in inspector.get_columns('generations')}
+        if 'status' not in columns:
+            print("Migrating generations: adding status column")
+            with engine.connect() as conn:
+                conn.execute(text("ALTER TABLE generations ADD COLUMN status VARCHAR DEFAULT 'completed'"))
+                conn.commit()
+                print("Added status column to generations")
+        if 'error' not in columns:
+            print("Migrating generations: adding error column")
+            with engine.connect() as conn:
+                conn.execute(text("ALTER TABLE generations ADD COLUMN error TEXT"))
+                conn.commit()
+                print("Added error column to generations")
+        if 'engine' not in columns:
+            print("Migrating generations: adding engine column")
+            with engine.connect() as conn:
+                conn.execute(text("ALTER TABLE generations ADD COLUMN engine VARCHAR DEFAULT 'qwen'"))
+                conn.commit()
+                print("Added engine column to generations")
+        # Re-read columns after engine migration (variable name shadows outer `engine`)
+        columns = {col['name'] for col in inspector.get_columns('generations')}
+        if 'model_size' not in columns:
+            print("Migrating generations: adding model_size column")
+            with engine.connect() as conn:
+                conn.execute(text("ALTER TABLE generations ADD COLUMN model_size VARCHAR"))
+                conn.commit()
+                print("Added model_size column to generations")
+
 
 def get_db():
     """Get database session (generator for dependency injection)."""