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/api_test.go

@@ -354,6 +354,13 @@ func (f *fakeRepo) VerifyEmailOTP(_ context.Context, userID, purpose, codeHash s
 		live.attempts++ // a typo costs an attempt but does not consume the code
 		return "", ErrOTPInvalid
 	}
+	// A DIFFERENT verified holder of the same address → ErrEmailTaken, code left
+	// live — mirrors PGRepo's guard + the users_verified_email_unique index.
+	for _, u := range f.staff {
+		if u.ID != userID && u.EmailVerified && strings.EqualFold(u.Email, live.email) {
+			return "", ErrEmailTaken
+		}
+	}
 	live.consumed = true
 	for _, u := range f.staff { // flip the user row verified (UPDATE users ...)
 		if u.ID == userID {

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

internal/api/handlers_email_otp.go

@@ -187,9 +187,11 @@ type emailOTPVerifyRequest struct {
 
 // handleEmailOTPVerify redeems a code for the caller (spec §B2, external app face).
 // Outcomes mirror the link-verify shape: an invalid/expired/mismatched code → 400
-// invalid_code, a locked code (too many wrong guesses) → 429 otp_locked, and on
-// success the user's email is written and email_verified flips true. The verified
-// address is echoed so the panel can render it.
+// invalid_code, a locked code (too many wrong guesses) → 429 otp_locked, an address
+// another account already proved → 409 email_taken (the code stays live — the
+// address, not the code, is the problem), and on success the user's email is
+// written and email_verified flips true. The verified address is echoed so the
+// panel can render it.
 func (a *API) handleEmailOTPVerify(w http.ResponseWriter, r *http.Request) {
 	p := principalFromContext(r.Context())
 	var req emailOTPVerifyRequest
@@ -211,6 +213,10 @@ func (a *API) handleEmailOTPVerify(w http.ResponseWriter, r *http.Request) {
 	case errors.Is(err, ErrOTPInvalid):
 		writeError(w, r, newError(http.StatusBadRequest, "invalid_code", "email code is invalid or expired"))
 		return
+	case errors.Is(err, ErrEmailTaken):
+		writeError(w, r, newError(http.StatusConflict, "email_taken",
+			"that email is already verified on another account; sign in with it or use another address"))
+		return
 	case err != nil:
 		writeError(w, r, err)
 		return

internal/api/handlers_email_otp_test.go

@@ -339,6 +339,20 @@ func TestEmailOTPVerifyRejections(t *testing.T) {
 			t.Fatalf("code = %d body %s, want 429 otp_locked", w.Code, w.Body.String())
 		}
 	})
+	t.Run("address proven elsewhere -> 409 email_taken, not consumed", func(t *testing.T) {
+		repo := newFakeRepo()
+		live(repo, "tk", otpCodeHash("123456"), time.Unix(1_700_000_600, 0), 0)
+		// Another account already proved the same address: login resolves accounts
+		// BY verified email, so the second proof must be refused.
+		repo.staff["other"] = &StaffUser{ID: "u2", Email: "[email protected]", EmailVerified: true}
+		w := do(mk(repo), "POST", "/api/v1/account/email/verify", `{"code":"123456"}`, nil)
+		if w.Code != http.StatusConflict || decodeErr(t, w) != "email_taken" {
+			t.Fatalf("code = %d body %s, want 409 email_taken", w.Code, w.Body.String())
+		}
+		if repo.otps["tk"].consumed {
+			t.Error("a taken address must not consume the code")
+		}
+	})
 }
 
 // TestEmailOTPBruteForceLockout drives the lockout end-to-end through the handler:

internal/api/pgrepo.go

@@ -695,7 +695,9 @@ func (p *PGRepo) CreateEmailOTP(ctx context.Context, id, userID, email, codeHash
 // never silently accepted, and a hash mismatch costs an attempt (UPDATE attempts+1)
 // without consuming the code — a typo must not burn a still-valid code. On a match
 // the code is consumed and the user row is flipped verified, returning the proven
-// address. ErrOTPInvalid / ErrOTPLocked are the only domain errors.
+// address — unless a DIFFERENT account already proved the same address, which is
+// ErrEmailTaken with the code left unconsumed (the address, not the guess, is the
+// problem). ErrOTPInvalid / ErrOTPLocked / ErrEmailTaken are the only domain errors.
 func (p *PGRepo) VerifyEmailOTP(ctx context.Context, userID, purpose, codeHash string, now time.Time) (string, error) {
 	tx, err := p.db.BeginTx(ctx, nil)
 	if err != nil {
@@ -738,12 +740,35 @@ func (p *PGRepo) VerifyEmailOTP(ctx context.Context, userID, purpose, codeHash s
 		return "", ErrOTPInvalid
 	}
 
+	// A DIFFERENT account may not also prove this address: the pre-session login
+	// door resolves accounts BY verified email (UserByEmail), so a second verified
+	// holder would make the identity ambiguous. The code is NOT consumed and no
+	// attempt is charged — the address, not the guess, is the problem. The check is
+	// the application-level counterpart of the users_verified_email_unique index
+	// (migration 0020), which catches a cross-user race that passes this SELECT.
+	var taken bool
+	if err := tx.QueryRowContext(ctx,
+		`SELECT EXISTS (SELECT 1 FROM users
+		 WHERE lower(email) = lower($1) AND email_verified = true AND id <> $2)`,
+		email, userID).Scan(&taken); err != nil {
+		return "", fmt.Errorf("check verified-email uniqueness: %w", err)
+	}
+	if taken {
+		return "", ErrEmailTaken
+	}
+
 	if _, err := tx.ExecContext(ctx,
 		`UPDATE email_otps SET consumed_at = $2 WHERE id = $1`, id, now); err != nil {
 		return "", fmt.Errorf("consume otp: %w", err)
 	}
 	if _, err := tx.ExecContext(ctx,
 		`UPDATE users SET email = $2, email_verified = true WHERE id = $1`, userID, email); err != nil {
+		// Lost the race the guarded SELECT above cannot serialise: the index rejects
+		// the second write, and it reads as the same answer the sequential path gives.
+		// The rollback undoes the OTP consumption with it, so the code stays live.
+		if isUniqueViolation(err) {
+			return "", ErrEmailTaken
+		}
 		return "", fmt.Errorf("mark email verified: %w", err)
 	}
 	if err := tx.Commit(); err != nil {

internal/api/repo.go

@@ -309,7 +309,10 @@ type Repo interface {
 	// mismatch increments attempts and returns ErrOTPInvalid WITHOUT consuming the
 	// code (so a typo does not burn it). On a match the code is consumed and the
 	// user row is flipped to email=<the proven address>, email_verified=true; the
-	// proven email is returned. now is the API clock so expiry is testable.
+	// proven email is returned — unless a DIFFERENT account has already proven the
+	// same address, which is ErrEmailTaken with the code left unconsumed (the
+	// address, not the code, is the problem). now is the API clock so expiry is
+	// testable.
 	//
 	// This is the ONBOARDING primitive: verifying the code is the moment the address
 	// becomes proven, so the write is load-bearing. The pre-session LOGIN door must
@@ -490,7 +493,7 @@ type Repo interface {
 	// (email_verified true), so a merely-asserted or unverified address never
 	// resolves to a session-mintable identity — an attacker cannot claim someone
 	// else's login by typing their email. Matching is on lower(email) to align with
-	// the users_verified_email_unique partial index (migration 0010), which
+	// the users_verified_email_unique partial index (migration 0020), which
 	// guarantees at most one verified row per normalized address, so the result is
 	// unambiguous. A player (role='user') row resolves too — email-first login is
 	// passwordless and role-agnostic here; the door that consumes this result decides