Fix model registration + add long-form generation support

- Fix critical bug: register_all_adapters() now called in main.py
- Add generate_long() method to MusicGen adapter for continuation-based
  extended tracks (up to 5 minutes)
- Add long-form checkbox in UI that unlocks duration slider to 300s
- Update GenerationService to route to generate_long when duration > 30s
- Update BatchProcessor to support long_form parameter

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

main.py

@@ -17,6 +17,7 @@ from src.services.batch_processor import BatchProcessor
 from src.services.project_service import ProjectService
 from src.storage.database import Database
 from src.ui.app import create_app
+from src.models import register_all_adapters
 
 
 # Configure logging
@@ -56,6 +57,10 @@ async def initialize_services():
         idle_timeout_minutes=settings.idle_unload_minutes,
     )
 
+    # Register all model adapters
+    logger.info("Registering model adapters...")
+    register_all_adapters(model_registry)
+
     # Initialize services
     logger.info("Initializing services...")
     generation_service = GenerationService(