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/api/errors.go

@@ -51,7 +51,8 @@ var (
 	// the handler answers 403 (wrong door) rather than 409 (already linked).
 	ErrPlayerBindForbidden = errors.New("bind code belongs to a staff account")
 	// ErrEmailTaken means a verified email would collide with another account's
-	// already-verified address (spec §B email-first login foundation, migration 0010).
+	// already-verified address (spec §B email-first login foundation; the
+	// users_verified_email_unique index ships in migration 0020).
 	// VerifyEmailOTP returns it — WITHOUT consuming the code, since the address, not
 	// the code, is the problem — when a DIFFERENT user has already proven the same
 	// address case-insensitively. It is the clean, application-level counterpart of