Use handler database for person referral lookup

[cybercraftsolutionsllc]

Summary

• resolve referred_by UUIDs through the injected people handler database instead of the global database
• add CreatePerson coverage proving a referral UUID stores the referrer person ID on the new person

Addresses #1484.

Validation

• go test ./routes -run '^TestPeopleRoutes$' -count=1
• git diff --check

Attempted but currently blocked by existing stale mocks:

• go test ./handlers -run '^TestCreatePerson$/should_create_user_with_referred_by_person_id_from_referral_uuid$' -count=1
• go test ./handlers -run '^$' -count=1

Both handler commands fail before running this test because mocks.Database is missing GetBountyByUnlockCode, matching the existing repo-wide handler test compile blocker.