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/user-profile/data/user-profile.repository.ts

@@ -640,4 +640,72 @@ export class UserProfileRepository {
       client.release();
     }
   }
+
+  /**
+   * Get total count of active vehicles across all users
+   * Used by admin stats endpoint for dashboard widget
+   */
+  async getTotalVehicleCount(): Promise<number> {
+    const query = `
+      SELECT COUNT(*) as total
+      FROM vehicles
+      WHERE is_active = true
+        AND deleted_at IS NULL
+    `;
+
+    try {
+      const result = await this.pool.query(query);
+      return parseInt(result.rows[0]?.total || '0', 10);
+    } catch (error) {
+      logger.error('Error getting total vehicle count', { error });
+      throw error;
+    }
+  }
+
+  /**
+   * Get total count of active users
+   * Used by admin stats endpoint for dashboard widget
+   */
+  async getTotalUserCount(): Promise<number> {
+    const query = `
+      SELECT COUNT(*) as total
+      FROM user_profiles
+      WHERE deactivated_at IS NULL
+    `;
+
+    try {
+      const result = await this.pool.query(query);
+      return parseInt(result.rows[0]?.total || '0', 10);
+    } catch (error) {
+      logger.error('Error getting total user count', { error });
+      throw error;
+    }
+  }
+
+  /**
+   * Get vehicles for a user (admin view)
+   * Returns only year, make, model for privacy
+   */
+  async getUserVehiclesForAdmin(auth0Sub: string): Promise<Array<{ year: number; make: string; model: string }>> {
+    const query = `
+      SELECT year, make, model
+      FROM vehicles
+      WHERE user_id = $1
+        AND is_active = true
+        AND deleted_at IS NULL
+      ORDER BY year DESC, make ASC, model ASC
+    `;
+
+    try {
+      const result = await this.pool.query(query, [auth0Sub]);
+      return result.rows.map(row => ({
+        year: row.year,
+        make: row.make,
+        model: row.model,
+      }));
+    } catch (error) {
+      logger.error('Error getting user vehicles for admin', { error, auth0Sub });
+      throw error;
+    }
+  }
 }