Plan Token Endpoint

Issue a signed plan claim. The caller is already authenticated via the session bearer token; we read their plan_code from the database (NOT from the request body — that's the whole point of signing it). 24h TTL by default; configurable via OMICOS_PLAN_TOKEN_TTL_SECONDS for staging.

Auth resolution is inlined here (rather than Depends(current_user)) so this endpoint can attach a machine-readable code to its 401/403 bodies. omicos-core's plan_token renewer maps any 401/403 from this route to RenewReason::SessionExpired purely on status code (see the cross-repo contract); the code:"session_expired" field is an additive diagnostic so SPAs/clients can match without status-only inference. The 503 signing/key-failure path below stays as-is and core maps it to RenewReason::ServerSigningError. The HTTP status codes and detail strings are UNCHANGED from the shared current_user dependency, so behavior for status-only consumers is identical.