An app host's /__forge/ door shows what is serving: the version, since when, and the last three releases #5978

From any app you should know what you are looking at, and how new it is.

Done when:

• [ ] On production, the door (src/apps/door.ts) shows Serving <version> since <time> and the three releases before it, each with its date and a link to its commit or release on the forge. They are read from the forge's own release record for this app.
• [ ] On a preview, it shows Preview of #N at <short sha>, deployed <time> instead.
• [ ] It is a named Server-Timing phase, never git on render.
• [ ] Tests: production with four releases shows the newest and three more, a preview shows its line, and an app never released says not released yet. Do not touch src/http/mcp.ts, src/contract/openapi.ts or docs/DESIGN.md.