An OIDC login now resolves the account by the durable (issuer, sub)
pair instead of the email claim. First contact stamps the pair — by
auto-provision or a one-time link against an existing account matched
by verified email — and every later login matches on it alone, so an
IdP-side email change refreshes the display email but can never
re-point the login at a different account. An email that matches an
account already bound to a different identity is rejected and audited
(auth.oidc.link_conflict), never silently re-bound.
The email gate tightens with it: linking and provisioning now require
an explicit email_verified: true — an absent claim is a reject
(oidc_email_unverified) rather than a benefit of the doubt, since an
IdP that does not assert verification cannot anchor the account match.
Provisioned accounts get a lowercased email and the same server-default
timezone resolution the register route uses.
Migration 0240 adds the nullable users.oidc_issuer / users.oidc_sub
columns with a composite unique index.
