Queue depths, for a scraper

Prometheus text format — the exposition every scraper already speaks, so nobody has to write one.\n\n**Not public, unlike `/health` and `/ready`.** Those answer *"can I reach you"*, which a partner may ask without a credential. Queue depths describe our internal state, and a stuck submission queue is a business fact rather than a liveness one, so this sits on `projects:write` — the ops scope no partner key is ever issued and which is not delegable.\n\nReports the depth of the three row-as-queue tables the product runs on: notifications, webhook deliveries and government submissions. **Not request latency** — a request counter would be easy to add and would not answer the question the readiness review asked, which was how anyone would notice that a partner had stopped receiving webhooks or that email had stopped being queued.