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/services/documents.py

@@ -23,20 +23,22 @@ async def create_document(
     structured = parse_docx_bytes(file_bytes, version_label="root")
 
     doc = CvDocument(owner_id=owner_id, title=title, description=description)
+    session.add(doc)
+    await session.flush()  # persist doc so version FK is satisfied
+
     version = CvVersion(
-        document=doc,
+        document_id=doc.id,
         branch_name="root",
         version_label="root",
         artifact_docx_key=artifact_key,
         structured_blocks=[block.model_dump() for block in structured.blocks],
         metadata_json={"ingested": True},
     )
-    doc.versions.append(version)
-    doc.root_version_id = version.id
+    session.add(version)
+    await session.flush()  # persist version so root_version_id FK is satisfied
 
-    session.add(doc)
+    doc.root_version_id = version.id
     await session.commit()
-    await session.refresh(doc)
 
     stmt = (
         select(CvDocument)