fix(auth): preserve NIP-OA owner for direct members

[cmyk]

Summary

On membership-enforced relays, a directly admitted agent could authenticate successfully but lose the owner asserted by its valid NIP-OA tag.

Direct membership admission returns without an owner because membership alone does not establish ownership. The AUTH handler previously attempted NIP-OA owner extraction only on open relays, so the direct-member path skipped extraction entirely. The owner mapping was therefore not materialized, the observer cache was not populated, and auth_ctx.agent_owner_pubkey remained unset for later owner-authorized behavior.

This change supplements only a missing admission owner with the existing cryptographically verified NIP-OA resolver, then reuses the existing materialization path. It does not change membership admission or observer authorization.

Security properties are preserved:

• an owner already returned by admission remains authoritative;
• NIP-OA signatures are verified against the authenticated agent key before an owner is returned;
• existing owner mappings remain first-write-wins;
• conflicting mappings fail closed;
• the auth context is populated only after the stored owner is confirmed to match.

Related issue

No open duplicate issue or PR found.

Related ownership and membership work:

• fix: unify NIP-OA relay membership enforcement across all ingress paths #490 unified NIP-OA relay-membership enforcement across ingress paths.
• feat(relay): allow agent owners to edit/manage agent-owned content #1403 uses the materialized NIP-OA ownership relationship for owner-authorized agent behavior.

Testing

Regression coverage exercises the real handle_auth boundary with PostgreSQL:

• a direct relay member with a valid signed NIP-OA tag materializes the owner mapping, populates the observer cache, and sets auth_ctx.agent_owner_pubkey;
• a pre-existing conflicting owner remains unchanged and the conflicting owner is not placed in the auth context;
• absent and invalid tags preserve ordinary direct membership without an owner;
• an owner returned by admission takes precedence over a conflicting tag.

Executed red/green proof:

• RED: with only the production owner-resolution hunk temporarily restored to its pre-fix form, the direct-member regression failed because auth_ctx.agent_owner_pubkey remained None;
• GREEN: after restoring the fix, both PostgreSQL-backed AUTH regressions passed.

Final verification:

• cargo test -p buzz-relay handlers::auth -- --include-ignored — 8 passed, 0 failed
• cargo clippy -p buzz-relay --tests -- -D warnings — passed
• cargo fmt --check — passed
• git diff --check — passed

No UI behavior changes; screenshots are not applicable.