Document staticGtfsHash and metadata.json on the wiki #1859

#1846 added staticGtfsHash to /api/v2/metadata.json and to /healthz?verbose=true, and made both return 503 when reading import_metadata fails (a database with no import yet still returns 200 with the static fields omitted). The wiki doesn't cover any of this yet.

healthz.md needs staticGtfsHash in its dataFreshness schema (hex sha256 of the raw static zip, omitted before the first import) and a new extension for the verbose=true 503 (status: "unavailable", detail: "reading data freshness failed").

/api/v2/metadata.json is listed in Maglev-1.0-Endpoint-Scope but has no page of its own. Watchdog now relies on its shape to detect when Maglev loads a new static bundle. The page should cover the API key requirement, the 30s Cache-Control, the fields staticGtfsLastUpdated, staticGtfsHash and realtimeFeeds (always an object, possibly empty), and the 503 cases.