Phoenix REST API

Endpoints: src/phoenix/server/api/routers/v1/. Read the relevant reference.

Checklist — run before committing any endpoint change

1. make openapi — regenerate schema + client types, commit all generated files
2. Add endpoint to the correct list in tests/integration/_helpers.py:
   • GET → _COMMON_RESOURCE_ENDPOINTS
   • Admin-only → _ADMIN_ONLY_ENDPOINTS
   • POST/PUT/DELETE → _VIEWER_BLOCKED_WRITE_OPERATIONS
   • Path format: use fake-id-{} for path params, test-tag for tag/name params (these are normalized by _ensure_endpoint_coverage_is_exhaustive)
3. make lint-python — fix any lint errors before committing