feat(metadata): implement metadata providers and initialize metadata service

- Added ModelMetadataProvider and CivitaiModelMetadataProvider for handling model metadata.
- Introduced SQLiteModelMetadataProvider for SQLite database integration.
- Created metadata_service.py to initialize and configure metadata providers.
- Updated CivitaiClient to register as a metadata provider.
- Refactored download_manager to use the new download_file method.
- Added SQL schema for models, model_versions, and model_files.
- Updated requirements.txt to include aiosqlite.

py/services/metadata_service.py

@@ -0,0 +1,33 @@
+import os
+import logging
+from .model_metadata_provider import ModelMetadataProviderManager, SQLiteModelMetadataProvider
+
+logger = logging.getLogger(__name__)
+
+async def initialize_metadata_providers():
+    """Initialize and configure all metadata providers"""
+    provider_manager = await ModelMetadataProviderManager.get_instance()
+    
+    # Use hardcoded SQLite DB path if not set in settings
+    db_path = os.path.join(
+        os.path.dirname(os.path.dirname(os.path.dirname(__file__))),
+        'civitai', 'civitai.sqlite'
+    )
+    if db_path and os.path.exists(db_path):
+        try:
+            sqlite_provider = SQLiteModelMetadataProvider(db_path)
+            provider_manager.register_provider('sqlite', sqlite_provider)
+            logger.info(f"SQLite metadata provider registered with database: {db_path}")
+        except Exception as e:
+            logger.error(f"Failed to initialize SQLite metadata provider: {e}")
+    
+    return provider_manager
+
+async def get_metadata_provider(provider_name: str = None):
+    """Get a specific metadata provider or default provider"""
+    provider_manager = await ModelMetadataProviderManager.get_instance()
+    
+    if provider_name:
+        return provider_manager._get_provider(provider_name)
+    
+    return provider_manager._get_provider()