Refactor backend to support interface-definition driven model resources

- Introduce a new model structure for managing interface definitions and model resources, enhancing the backend's capability to handle various service integrations.
- Update the Makefile to reflect changes in database seeding and resource management commands.
- Remove the deprecated credentials management routes and replace them with a unified model registry API.
- Modify existing routes and schemas to align with the new model structure, ensuring seamless integration with the frontend.
- Enhance database seeding scripts to populate new model resources and their configurations.
- Update README documentation to reflect the new architecture and usage instructions for model resources and interface definitions.

backend/services/masking.py

@@ -25,3 +25,23 @@ def resolve_incoming_key(incoming: str | None, stored: str) -> str:
     if incoming is None or incoming == "" or is_masked(incoming):
         return stored
     return incoming
+
+
+def mask_secrets(value):
+    """Recursively mask every scalar in a model resource secrets object."""
+    if isinstance(value, dict):
+        return {key: mask_secrets(item) for key, item in value.items()}
+    if isinstance(value, list):
+        return [mask_secrets(item) for item in value]
+    return mask(str(value)) if value is not None else ""
+
+
+def merge_secrets(incoming: dict, stored: dict) -> dict:
+    """Merge secret fields while treating masked/empty values as keep-existing."""
+    result = dict(stored or {})
+    for key, value in (incoming or {}).items():
+        if isinstance(value, dict) and isinstance(result.get(key), dict):
+            result[key] = merge_secrets(value, result[key])
+        elif value not in (None, "") and not is_masked(str(value)):
+            result[key] = value
+    return result