Adds modelRegistry: per-provider explicit+discovered model sets, bare-id
and provider-prefixed resolution with sorted-name collision precedence,
group authorization pairing (allowsModel AND allowsProvider), and a
group-filtered OpenAI-compatible /v1/models catalog.
Discovery is lazy and throttled per provider: newGateway runs a
synchronous 5s-bounded warm fill for discovery-enabled providers
(non-fatal on error), and Gateway.ServeHTTP triggers a throttled,
inFlight-guarded background refresh per provider per interval
(default 1h, per-provider DiscoveryInterval parsed at construction).
A failed refresh keeps the previous discovered set (stale-while-error).
Wires GET /v1/models into ServeHTTP (auth required, 401 envelope
otherwise) and gives Gateway its adapters/registry fields, both built
in newGateway.
newModelRegistry's signature gained an error return beyond the task
brief's literal listing: an invalid DiscoveryInterval must be a
constructor error per the controller ruling, which the brief's bare
*modelRegistry signature could not express.
