feat(refserver): seed admin API key on first boot via REFSERVER_BOOTSTRAP_API_KEY
Breaks the chicken-and-egg of needing an admin key to create keys via the
API. On startup, if the ApiKey collection is empty, refserver seeds an
admin-scoped key from REFSERVER_BOOTSTRAP_API_KEY (idempotent; refuses if other
keys already exist). Lets the combined stack deploy refserver for the first time
without manual key insertion.