The classic Fitbit Web API authorises with PKCE (Authorization Code + S256),
so the connect route mints a code_verifier and the callback presents it on the
token exchange. Carry it on the short-lived FitbitOAuthState row.
Additive + nullable (migration 0191): a legacy in-flight row carries no
verifier, so the change is reversible and rerun-safe with no data loss.
