feat: add Backend Mode toggle to disable user self-service

Add a system-wide "Backend Mode" that disables user self-registration
and self-service while keeping admin panel and API gateway fully
functional. When enabled, only admin can log in; all user-facing
routes return 403.

Backend:
- New setting key `backend_mode_enabled` with atomic cached reads (60s TTL)
- BackendModeUserGuard middleware blocks non-admin authenticated routes
- BackendModeAuthGuard middleware blocks registration/password-reset auth routes
- Login/Login2FA/RefreshToken handlers reject non-admin when enabled
- TokenPairWithUser struct for role-aware token refresh
- 20 unit tests (middleware + service layer)

Frontend:
- Router guards redirect unauthenticated users to /login
- Admin toggle in Settings page
- Login page hides register link and footer in backend mode
- 9 unit tests for router guard logic
- i18n support (en/zh)

27 files changed, 833 insertions(+), 17 deletions(-)

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

backend/internal/server/middleware/backend_mode_guard.go

@@ -0,0 +1,51 @@
+package middleware
+
+import (
+	"strings"
+
+	"github.com/Wei-Shaw/sub2api/internal/pkg/response"
+	"github.com/Wei-Shaw/sub2api/internal/service"
+
+	"github.com/gin-gonic/gin"
+)
+
+// BackendModeUserGuard blocks non-admin users from accessing user routes when backend mode is enabled.
+// Must be placed AFTER JWT auth middleware so that the user role is available in context.
+func BackendModeUserGuard(settingService *service.SettingService) gin.HandlerFunc {
+	return func(c *gin.Context) {
+		if settingService == nil || !settingService.IsBackendModeEnabled(c.Request.Context()) {
+			c.Next()
+			return
+		}
+		role, _ := GetUserRoleFromContext(c)
+		if role == "admin" {
+			c.Next()
+			return
+		}
+		response.Forbidden(c, "Backend mode is active. User self-service is disabled.")
+		c.Abort()
+	}
+}
+
+// BackendModeAuthGuard selectively blocks auth endpoints when backend mode is enabled.
+// Allows: login, login/2fa, logout, refresh (admin needs these).
+// Blocks: register, forgot-password, reset-password, OAuth, etc.
+func BackendModeAuthGuard(settingService *service.SettingService) gin.HandlerFunc {
+	return func(c *gin.Context) {
+		if settingService == nil || !settingService.IsBackendModeEnabled(c.Request.Context()) {
+			c.Next()
+			return
+		}
+		path := c.Request.URL.Path
+		// Allow login, 2FA, logout, refresh, public settings
+		allowedSuffixes := []string{"/auth/login", "/auth/login/2fa", "/auth/logout", "/auth/refresh"}
+		for _, suffix := range allowedSuffixes {
+			if strings.HasSuffix(path, suffix) {
+				c.Next()
+				return
+			}
+		}
+		response.Forbidden(c, "Backend mode is active. Registration and self-service auth flows are disabled.")
+		c.Abort()
+	}
+}