fix(auth): enforce the verified-email uniqueness that email login assumes

The design has claimed since migration 0010 that at most one account can hold
a PROVEN email address, with ErrEmailTaken as the 409 a second verifier sees.
Neither half ever shipped: no migration created users_verified_email_unique,
and VerifyEmailOTP had no guard at all — the sentinel was defined but never
returned, so two accounts could both verify one address. The damage is not
cosmetic: the pre-session login resolves accounts BY verified email, so the
duplicate decided which identity a mailed sign-in code belonged to.

- Migration 0020 creates the partial unique index (lower(email) WHERE
  email_verified) the comments have been citing — the database-level backstop.
- VerifyEmailOTP now refuses the take-over with ErrEmailTaken BEFORE consuming
  the code (the address, not the code, is the problem), charges no attempt,
  and maps a lost cross-user race (unique violation) to the same answer.
- The verify handler answers 409 email_taken instead of a generic 500.

Covered by the pgint suite (sequential double-verify refused with the code
still live, a direct duplicate write still loses to the index, the refused
account can still prove its own address) and a hermetic 409 case.

internal/store/migrations/0020_verified_email_unique.sql

@@ -0,0 +1,15 @@
+-- The verified-email uniqueness that the email-first login design has claimed
+-- since 0010 but no migration ever actually created: at most one account may
+-- hold a PROVEN address (email_verified), compared case-insensitively, so the
+-- pre-session login door (UserByEmail on lower(email)) resolves at most one
+-- identity. VerifyEmailOTP's ErrEmailTaken guard is the application-level
+-- counterpart; this index is the database-level backstop behind it, so a
+-- cross-user race that passes the guard still cannot write a second verified
+-- row (it loses to a unique violation instead).
+--
+-- If an upgrade meets a database where the missing guard already produced two
+-- verified rows for one address, CREATE UNIQUE INDEX refuses to build and names
+-- the duplicated key in DETAIL. That is deliberate — no silent "unverify the
+-- loser" surgery: only a human can say which holder keeps the address.
+CREATE UNIQUE INDEX users_verified_email_unique ON users (lower(email))
+  WHERE email_verified;