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/postgres_store.go

@@ -29,6 +29,7 @@ func (p *Postgres) CreateUser(ctx context.Context, u *User) error {
 		return fmt.Errorf("invalid role")
 	}
 	u.Username = NormalizeUsername(u.Username)
+	u.Email = NormalizeEmail(u.Email)
 	if u.ID == "" {
 		u.ID = uuid.NewString()
 	}
@@ -69,6 +70,7 @@ func (p *Postgres) CreateUser(ctx context.Context, u *User) error {
 		Name:         u.Name,
 		PasswordHash: u.PasswordHash,
 		Role:         string(role),
+		Email:        u.Email,
 		CreatedAt:    u.CreatedAt,
 	}); err != nil {
 		return mapUniqueViolation(err)