Grant First Call

Redeem a 'first call on us' grant token for the target api_key.

REAL BUG FOUND+FIXED (2026-09-01, real recipient hit it live -- a signed grant token minted and posted to a collaborator returned {"detail":"Not Found"} on redemption): this endpoint was only ever implemented on bridge.py (port 8081, internal-only, never proxied by nginx's location / block which forwards everything to app.py on 8000). Every other bridge-only endpoint actually reachable from the public API (/register, /referral/info, /topup, /withdraw, etc.) has its own thin httpx-proxy route here in routes/credit.py -- this one was simply missing, so a real, publicly-documented feature (see routes/pages.py's quickstart copy, which has always described "redeem via /grant_first_call") 404'd for every external caller since it was built. Same proxy pattern as /register above: forward the status code, don't launder an upstream error into a 200 (the exact class of bug 2026-07-30's /register fix already closed once).