Refresh core_nt OpenAPI last_updated

Motivation

After an NCBI Direct core_nt generation was promoted in Azure Storage, GET /api/aks/openapi/databases/core_nt returned the new generation ID and counts but retained last_updated from the stable legacy NCBI metadata blob. Consumers therefore saw a new snapshot paired with an older release timestamp.

User-Facing Change

The database detail API now reports last_updated from the active generation's source_release_at. Older metadata records that store the release timestamp only at the top level remain supported. Databases without an active generation keep the existing metadata-blob value.

API / IaC Diff Summary

• Updated the existing last_updated value in the OpenAPI-compatible database detail response; no response field was added or removed.
• Kept the active generation as the authoritative source and retained a top-level compatibility fallback.
• No IaC, authentication, RBAC, Storage network, or persistence schema changed.

Validation Evidence

• uv run pytest -q api/tests/test_aks_openapi_databases.py passed: 32 tests.
• Full backend suite passed: 6,016 tests with 4 fixture-dependent skips.
• Ruff, the mypy debt ratchet, the 242-operation OpenAPI contract, generated TypeScript API types, and strict MkDocs build passed.
• Local API smoke passed: 27 of 27 checks.
• Regression coverage verifies both the active-generation timestamp and the legacy top-level fallback.