fix: keep one active email OTP per account

frontend/db/migrations/20260810024500_unique_verification_identifiers.sql

@@ -0,0 +1,18 @@
+lock table identity.verifications in share row exclusive mode;
+
+with ranked as (
+  select
+    id,
+    row_number() over (
+      partition by identifier
+      order by created_at desc, id desc
+    ) as position
+  from identity.verifications
+)
+delete from identity.verifications verification
+using ranked
+where verification.id = ranked.id
+  and ranked.position > 1;
+
+create unique index if not exists identity_verifications_identifier_key
+  on identity.verifications (identifier);