Wire up deployment metadata: build commit, number, and server start time; v2.0.30

- Dockerfile: expose BUILD_COMMIT and BUILD_NUMBER env vars from build args
  (was COMMIT_SHA only; server was reading BUILD_COMMIT so it always fell through to null)
- CI: pass github.run_number as BUILD_NUMBER build arg
- state.js: record serverStartedAt at module load so deployedAt always shows
  when the current container instance came up, even without an env var
- admin-content.js: fall back COMMIT_SHA → BUILD_COMMIT for older images,
  use state.serverStartedAt when DEPLOYED_AT env var not set

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

.github/workflows/docker-publish.yml

@@ -63,3 +63,4 @@ jobs:
           labels: ${{ steps.meta.outputs.labels }}
           build-args: |
             COMMIT_SHA=${{ github.sha }}
+            BUILD_NUMBER=${{ github.run_number }}