feat: add email notification service with SMTP support

- Create pkg/email package (sender, templates, types)
- SMTP client with TLS, rate limiting, async sending
- HTML email templates with GooSeek branding
- Integrate welcome + password reset emails in auth-svc
- Add limit warning emails (80%/100%) in llm-svc middleware
- Add space invite endpoint with email notification in thread-svc
- Add GetUserEmail helper in JWT middleware
- Add SMTP config to .env, config.go, K8s configmap

Made-with: Cursor

backend/internal/usage/repository.go

@@ -156,6 +156,17 @@ func (r *Repository) GetUsageHistory(ctx context.Context, userID string, days in
 	return metrics, nil
 }
 
+func (r *Repository) GetUserEmail(ctx context.Context, userID string) (string, error) {
+	var emailAddr string
+	err := r.db.QueryRowContext(ctx,
+		"SELECT email FROM auth_users WHERE id = $1", userID,
+	).Scan(&emailAddr)
+	if err != nil {
+		return "", err
+	}
+	return emailAddr, nil
+}
+
 func (r *Repository) CheckLLMLimits(ctx context.Context, userID, tier string) (bool, string) {
 	usage, err := r.GetTodayUsage(ctx, userID)
 	if err != nil {