Add admin subdomain redirect for admin.myhoneydue.com

When ADMIN_HOST is set, redirects root "/" to "/admin/" so
admin.myhoneydue.com works without needing the /admin path suffix.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

internal/admin/routes.go

@@ -456,6 +456,19 @@ func setupAdminProxy(router *echo.Echo) {
 		adminURL = "http://127.0.0.1:3001"
 	}
 
+	// Admin subdomain (e.g. admin.myhoneydue.com) — redirect root to /admin/
+	adminHost := os.Getenv("ADMIN_HOST")
+	if adminHost != "" {
+		router.Use(func(next echo.HandlerFunc) echo.HandlerFunc {
+			return func(c echo.Context) error {
+				if c.Request().Host == adminHost && c.Request().URL.Path == "/" {
+					return c.Redirect(http.StatusMovedPermanently, "/admin/")
+				}
+				return next(c)
+			}
+		})
+	}
+
 	target, err := url.Parse(adminURL)
 	if err != nil {
 		return