Capture which Yggdrasil authenticated an in-game UUID when a link code is
minted (spec §10 dual-Yggdrasil) and copy it onto the durable account_links
row at verify. The value originates in-game — the web verify side never sees
the authentication — so it threads through account_link_codes, mirroring how
mc_uuid (not user_id) lives on a code.
- migration 0005: add link_auth_source enum + auth_source column on both
account_link_codes and account_links; DEFAULT 'mojang' backfills existing
rows and sets the Mojang-priority default for a mint that omits the field
- mint validates an explicit auth_source (unknown value -> 400); verify
surfaces it in the 200 body and refreshes it on idempotent re-verify
