Show OpenAPI last-modified timestamp in /docs

[Mayank11104]

Changes

Change Type: Added

Change Category: Documentation
Changelog Entry:
Added info.x-lastModified (ISO-8601 UTC timestamp) to the generated OpenAPI schema and exposed it in Swagger UI /docs by appending it to info.description.

How to Test

1. Run the stack (docker compose).
2. Open http://localhost:8000/openapi.json and verify info.x-lastModified exists.
3. Open http://localhost:8000/docs and verify "Last modified: " appears in the API description block.
4. Restart/recreate the apiserver container and confirm the timestamp updates.

Checklist

• Tests have been added or updated to reflect the changes, or their absence is explicitly explained.
• Documentation has been added or updated to reflect the changes, or their absence is explicitly explained. (Shown in /docs UI.)
• A self-review has been conducted.
• All CI checks pass before pinging a reviewer, or provide an explanation if they do not.
• The PR title matches the changelog entry's one-line description.

Related Issues

Closes #572