/metrics adds the delivery series: pulls merged, releases, ask-to-live seconds, red-main seconds #3750

Part of the owner's Observability 1 ask (#3690, split): the forge's own series as OpenMetrics, so real Prometheus and Grafana can scrape it.

Done:

• [ ] /metrics adds four families: forge_pulls_merged_total (by repository), forge_releases_total, forge_ask_to_live_seconds (a histogram from an issue opening to its pull's release), and forge_main_red_seconds_total (time the default branch's check spent failing, by repository). Each is read from the tables that already hold these facts.
• [ ] Private repositories appear under their id only, never their name.
• [ ] docs/METRICS.md lists all four. A test makes each one move: merge a pull, cut a release, fail and then fix main.

Files: src/metrics/forge.ts, docs/METRICS.md, test/metrics-delivery.test.ts (new). Risk: a slow read; each family is one indexed query.