Introspect the calling API key: user identity, owning app, scope, USDC balance and remaining spend allowance — one call.

Everything an API-key client needs to know about itself in one call.

Returned by GET /apps/me. Avoids the older multi-step dance of GET /user + GET /apps/{id} + GET /balance for callers (e.g. the CLI) that only have an X-Api-Key and want to introspect.