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/web/auth.go

@@ -129,14 +129,21 @@ func (s *Server) handleRegister(w http.ResponseWriter, r *http.Request) {
 		return
 	}
 	username := strings.TrimSpace(r.PostFormValue("username"))
+	emailRaw := r.PostFormValue("email")
 	password := r.PostFormValue("password")
 	setupSecret := r.PostFormValue("setup_secret")
-	p := authPage{page: s.basePage(r, "Create account"), Username: username}
+	p := authPage{page: s.basePage(r, "Create account"), Username: username, Email: strings.TrimSpace(emailRaw)}
 	if !usernameRe.MatchString(username) {
 		p.Error = "Username must be 3–20 letters, numbers, or underscores."
 		s.exec(w, "register", p)
 		return
 	}
+	email, emailErr := store.ValidateEmail(emailRaw)
+	if emailErr != "" {
+		p.Error = emailErr
+		s.exec(w, "register", p)
+		return
+	}
 	if ok, msg := passwordValid(password); !ok {
 		p.Error = msg
 		s.exec(w, "register", p)
@@ -153,6 +160,7 @@ func (s *Server) handleRegister(w http.ResponseWriter, r *http.Request) {
 	}
 	u := &store.User{
 		Username:     username,
+		Email:        email,
 		PasswordHash: string(hash),
 		Role:         role,
 	}
@@ -162,6 +170,11 @@ func (s *Server) handleRegister(w http.ResponseWriter, r *http.Request) {
 			s.exec(w, "register", p)
 			return
 		}
+		if errors.Is(err, store.ErrDuplicateEmail) {
+			p.Error = "That email is already registered."
+			s.exec(w, "register", p)
+			return
+		}
 		log.Printf("register create: %v", err)
 		http.Error(w, "could not create account", http.StatusInternalServerError)
 		return