feat(fm): list our own zones, and answer relayed agreement calls as ProblemDetails

OEG now relays /internal/agreements to the portal, so those routes join the
proxied set: a bad body comes back as a 400 ProblemDetails that OEG can pass
on, not FastAPI's 422.

GET /internal/local-zones lists the zones an agreement can offer: SRM's zones
minus the ones a partner offers us. Zone offers already filtered those out;
both now share local_zones().

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>