The link flow was stubbed: consume_link_nonce always returned false and no nonce
issuer existed, so nostr_login worked only for already-linked npubs and nothing
could link one. Complete it:
- GET /auth/nostr/link-challenge (authenticated) issues a single-use nonce via
the existing challenge store, bound to the caller (subject) + the nostr_link
purpose, 5-minute TTL.
- nostr_link now consumes it atomically (DELETE … RETURNING, so a replay loses
the race), cross-checks the nonce was issued to THIS user, then verifies the
signed-action proof (unchanged) and stores the npub.
- Route registered + OpenAPI regenerated (no drift).
tests/nostr_auth.rs: pins the empty-body descriptor hash as a cross-impl KAT (the
wallet binds the identical value), and covers a full challenge→link round-trip
plus replay and wrong-user-nonce rejection.
