Notify question authors via Resend when an admin posts the first answer.

Require email at registration (editable on profile), add a Resend mailer with idempotent first-answer sends, and keep answer saves independent of delivery.

internal/store/vote.go

@@ -5,6 +5,7 @@ import (
 	"database/sql"
 	"errors"
 	"fmt"
+	"strings"
 
 	"github.com/jackc/pgx/v5/pgconn"
 
@@ -14,6 +15,9 @@ import (
 // ErrDuplicateUsername is returned when inserting a username that already exists.
 var ErrDuplicateUsername = errors.New("username taken")
 
+// ErrDuplicateEmail is returned when inserting/updating an email that already exists.
+var ErrDuplicateEmail = errors.New("email taken")
+
 // ErrHiddenOrMissing is returned when voting on a hidden or unknown question.
 var ErrHiddenOrMissing = errors.New("question not votable")
 
@@ -58,6 +62,9 @@ func Vote(ctx context.Context, db *sql.DB, userID, questionID string, value int)
 func mapUniqueViolation(err error) error {
 	var pgErr *pgconn.PgError
 	if errors.As(err, &pgErr) && pgErr.Code == "23505" {
+		if strings.Contains(strings.ToLower(pgErr.ConstraintName), "email") {
+			return ErrDuplicateEmail
+		}
 		return ErrDuplicateUsername
 	}
 	return err