feat(metadata): implement fallback provider strategy for deleted models

Refactor metadata sync service to use a prioritized provider fallback system when handling deleted CivitAI models. The new approach:

1. Attempts civarchive_api provider first for deleted models
2. Falls back to sqlite provider if archive DB is enabled
3. Maintains existing default provider behavior for non-deleted models
4. Tracks provider attempts and errors for better debugging

This improves reliability when fetching metadata for deleted models by trying multiple sources before giving up, and provides clearer error messages based on which providers were attempted.

py/utils/models.py

@@ -25,6 +25,7 @@ class BaseModelMetadata:
     favorite: bool = False      # Whether the model is a favorite
     exclude: bool = False       # Whether to exclude this model from the cache
     db_checked: bool = False    # Whether checked in archive DB
+    metadata_source: Optional[str] = None  # Last provider that supplied metadata
     last_checked_at: float = 0  # Last checked timestamp
     _unknown_fields: Dict[str, Any] = field(default_factory=dict, repr=False, compare=False)  # Store unknown fields