Fix document loading: circular FK, patch validation 500, and token expiry redirect

- documents.py: fix root_version_id never being saved due to SQLAlchemy
  deferring the circular FK (CvDocument↔CvVersion). Use flush-based approach:
  flush doc, flush version, then set root_version_id before final commit.
- config.py: add validator to coerce empty MINIO_ENDPOINT string to None,
  preventing boto3 ValueError: Invalid endpoint at startup.
- versions.py: catch PatchValidationError and return 422 instead of 500
  when a patch violates ATS guard rules.
- api.ts: on 401, clear stale OIDC cookies and redirect to /login instead
  of showing the "Failed to load documents" error.

https://claude.ai/code/session_01KKbzWYz8fLyG2qcwiDZ8fy

apps/backend/fastapi/app/core/config.py

@@ -57,6 +57,13 @@ class Settings(BaseSettings):
             return [origin.strip() for origin in value.split(",") if origin.strip()]
         return value
 
+    @field_validator("storage_endpoint_url", mode="before")
+    @classmethod
+    def _empty_endpoint_to_none(cls, value):
+        if isinstance(value, str) and not value.strip():
+            return None
+        return value
+
 
 @lru_cache(maxsize=1)
 def get_settings() -> Settings: