feat(invitation): send email notification when inviting a user

Uses the existing Resend email service to notify invitees.
Email includes inviter name, workspace name, and a link to the app.
Sent fire-and-forget in a goroutine to avoid blocking the API response.

server/internal/handler/invitation.go

@@ -134,11 +134,26 @@ func (h *Handler) CreateInvitation(w http.ResponseWriter, r *http.Request) {
 	// Notify the invitee in real time if they are a registered user.
 	userID := requestUserID(r)
 	eventPayload := map[string]any{"invitation": resp}
+	var workspaceName string
 	if ws, err := h.Queries.GetWorkspace(r.Context(), parseUUID(workspaceID)); err == nil {
+		workspaceName = ws.Name
 		eventPayload["workspace_name"] = ws.Name
 	}
 	h.publish(protocol.EventInvitationCreated, workspaceID, "member", userID, eventPayload)
 
+	// Send invitation email (fire-and-forget).
+	if h.EmailService != nil && workspaceName != "" {
+		inviterName := email // fallback
+		if inviter, err := h.Queries.GetUser(r.Context(), requester.UserID); err == nil {
+			inviterName = inviter.Name
+		}
+		go func() {
+			if err := h.EmailService.SendInvitationEmail(email, inviterName, workspaceName); err != nil {
+				slog.Warn("failed to send invitation email", "email", email, "error", err)
+			}
+		}()
+	}
+
 	writeJSON(w, http.StatusCreated, resp)
 }
 

server/internal/service/email.go

@@ -3,6 +3,7 @@ package service
 import (
 	"fmt"
 	"os"
+	"strings"
 
 	"github.com/resend/resend-go/v2"
 )
@@ -52,3 +53,36 @@ func (s *EmailService) SendVerificationCode(to, code string) error {
 	_, err := s.client.Emails.Send(params)
 	return err
 }
+
+// SendInvitationEmail notifies the invitee that they have been invited to a workspace.
+func (s *EmailService) SendInvitationEmail(to, inviterName, workspaceName string) error {
+	// Build the app URL for the invitation — users will see pending invitations
+	// in the workspace switcher after logging in.
+	appURL := strings.TrimSpace(os.Getenv("FRONTEND_ORIGIN"))
+	if appURL == "" {
+		appURL = "https://app.multica.ai"
+	}
+
+	if s.client == nil {
+		fmt.Printf("[DEV] Invitation email to %s: %s invited you to %s — %s\n", to, inviterName, workspaceName, appURL)
+		return nil
+	}
+
+	params := &resend.SendEmailRequest{
+		From:    s.fromEmail,
+		To:      []string{to},
+		Subject: fmt.Sprintf("%s invited you to %s on Multica", inviterName, workspaceName),
+		Html: fmt.Sprintf(
+			`<div style="font-family: sans-serif; max-width: 480px; margin: 0 auto;">
+				<h2>You're invited to join %s</h2>
+				<p><strong>%s</strong> invited you to collaborate in the <strong>%s</strong> workspace on Multica.</p>
+				<p style="margin: 24px 0;">
+					<a href="%s" style="display: inline-block; padding: 12px 24px; background: #000; color: #fff; text-decoration: none; border-radius: 6px; font-weight: 500;">Open Multica</a>
+				</p>
+				<p style="color: #666; font-size: 14px;">Log in to accept or decline the invitation.</p>
+			</div>`, workspaceName, inviterName, workspaceName, appURL),
+	}
+
+	_, err := s.client.Emails.Send(params)
+	return err
+}