Add auth guards to sensitive mutating endpoints

[maxachis]

Summary

• require admin auth for proposal mutation endpoints that were missing protection
• require authenticated user auth for
• add regression test coverage to prevent future auth drift on these routes

Testing

• uv run pytest -q tests/automated/integration/api/test_sensitive_endpoint_auth_config.py

Closes #574

[josh-chamberlain]

hm...yikes! nice catch