feat: Add admin vehicle management and profile vehicles display (refs #11)

- Add GET /api/admin/stats endpoint for Total Vehicles widget
- Add GET /api/admin/users/:auth0Sub/vehicles endpoint for user vehicle list
- Update AdminUsersPage with Total Vehicles stat and expandable vehicle rows
- Add My Vehicles section to SettingsPage (desktop) and MobileSettingsScreen
- Update AdminUsersMobileScreen with stats header and vehicle expansion
- Add defense-in-depth admin checks and error handling
- Update admin README documentation

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

backend/src/features/admin/api/admin.routes.ts

@@ -102,6 +102,16 @@ export const adminRoutes: FastifyPluginAsync = async (fastify) => {
     handler: adminController.bulkReinstateAdmins.bind(adminController)
   });
 
+  // ============================================
+  // Admin Stats endpoint (dashboard widgets)
+  // ============================================
+
+  // GET /api/admin/stats - Get admin dashboard stats (total users, total vehicles)
+  fastify.get('/admin/stats', {
+    preHandler: [fastify.requireAdmin],
+    handler: usersController.getAdminStats.bind(usersController)
+  });
+
   // ============================================
   // User Management endpoints (subscription tiers, deactivation)
   // ============================================
@@ -118,6 +128,12 @@ export const adminRoutes: FastifyPluginAsync = async (fastify) => {
     handler: usersController.getUser.bind(usersController)
   });
 
+  // GET /api/admin/users/:auth0Sub/vehicles - Get user's vehicles (admin view)
+  fastify.get<{ Params: UserAuth0SubInput }>('/admin/users/:auth0Sub/vehicles', {
+    preHandler: [fastify.requireAdmin],
+    handler: usersController.getUserVehicles.bind(usersController)
+  });
+
   // PATCH /api/admin/users/:auth0Sub/tier - Update subscription tier
   fastify.patch<{ Params: UserAuth0SubInput; Body: UpdateTierInput }>('/admin/users/:auth0Sub/tier', {
     preHandler: [fastify.requireAdmin],