Implement Phase 1 admin dashboard backend

- Add database indexes for admin queries (role, isActive+role, createdAt, lastLoginAt)
- Update setUserActive to track disabledAt and disabledBy fields
- Add getPaginatedUsers function with search, filter, sort support
- Add updateUserRole function for promoting/demoting users
- Create admin routes with full user management API:
  - GET /api/admin/users - paginated list with search/filter/sort
  - GET /api/admin/users/:id - user details with recent activity
  - PATCH /api/admin/users/:id - enable/disable users
  - PATCH /api/admin/users/:id/role - promote/demote users
- Add validation for ObjectId format and self-modification prevention
- All 53 Phase 1 backend tests passing

models/user.js

@@ -73,15 +73,37 @@ async function findById(db, id) {
  * @param {Db} db - MongoDB database instance
  * @param {string|ObjectId} id - User's MongoDB _id
  * @param {boolean} isActive - New active status
+ * @param {string|ObjectId|null} adminId - Admin performing the action (required when disabling)
  * @returns {Promise<Object>} Update result
  */
-async function setUserActive(db, id, isActive) {
+async function setUserActive(db, id, isActive, adminId = null) {
   // Convert string to ObjectId if needed
   const objectId = typeof id === 'string' ? new ObjectId(id) : id;
 
+  let updateDoc;
+  if (isActive) {
+    // Enabling: clear disabledAt and disabledBy
+    updateDoc = {
+      $set: { isActive: true, disabledAt: null, disabledBy: null }
+    };
+  } else {
+    // Disabling: set disabledAt and disabledBy
+    // Convert string to ObjectId if needed, keep ObjectId as-is
+    const adminObjectId = adminId && typeof adminId === 'string'
+      ? new ObjectId(adminId)
+      : adminId;
+    updateDoc = {
+      $set: {
+        isActive: false,
+        disabledAt: new Date(),
+        disabledBy: adminObjectId
+      }
+    };
+  }
+
   const result = await db.collection(USER_COLLECTION).findOneAndUpdate(
     { _id: objectId },
-    { $set: { isActive } },
+    updateDoc,
     { returnDocument: 'after' }
   );
 
@@ -115,14 +137,117 @@ async function createIndexes(db) {
     { isActive: 1, lastLoginAt: -1 }
   );
 
+  // Index on role for admin queries
+  await collection.createIndex({ role: 1 });
+
+  // Compound index on isActive and role for filtered admin queries
+  await collection.createIndex({ isActive: 1, role: 1 });
+
+  // Index on createdAt for recent registrations
+  await collection.createIndex({ createdAt: -1 });
+
+  // Index on lastLoginAt for recently active users
+  await collection.createIndex({ lastLoginAt: -1 });
+
   console.log('User collection indexes created successfully');
 }
 
+/**
+ * Get paginated list of users with optional filtering and sorting
+ *
+ * @param {Db} db - MongoDB database instance
+ * @param {Object} options - Query options
+ * @param {number} options.page - Page number (1-indexed)
+ * @param {number} options.limit - Items per page (max 100)
+ * @param {string} options.search - Search term for name/email
+ * @param {string} options.status - Filter by status: 'all', 'active', 'disabled'
+ * @param {string} options.sort - Sort field: 'createdAt', 'lastLoginAt', 'loginCount'
+ * @param {string} options.order - Sort order: 'asc', 'desc'
+ * @returns {Promise<{users: Array, pagination: Object}>}
+ */
+async function getPaginatedUsers(db, options = {}) {
+  const page = Math.max(1, parseInt(options.page) || 1);
+  const limit = Math.min(100, Math.max(1, parseInt(options.limit) || 20));
+  const skip = (page - 1) * limit;
+
+  // Build filter
+  const filter = {};
+  if (options.search) {
+    const searchRegex = new RegExp(options.search, 'i');
+    filter.$or = [{ name: searchRegex }, { email: searchRegex }];
+  }
+  if (options.status === 'active') {
+    filter.isActive = true;
+  } else if (options.status === 'disabled') {
+    filter.isActive = false;
+  }
+
+  // Build sort
+  const sortField = ['createdAt', 'lastLoginAt', 'loginCount'].includes(options.sort)
+    ? options.sort
+    : 'createdAt';
+  const sortOrder = options.order === 'asc' ? 1 : -1;
+  const sort = { [sortField]: sortOrder };
+
+  const collection = db.collection(USER_COLLECTION);
+
+  // Execute queries in parallel
+  const [users, total] = await Promise.all([
+    collection.find(filter).sort(sort).skip(skip).limit(limit).toArray(),
+    collection.countDocuments(filter)
+  ]);
+
+  return {
+    users,
+    pagination: {
+      page,
+      limit,
+      total,
+      pages: Math.ceil(total / limit)
+    }
+  };
+}
+
+/**
+ * Update a user's role
+ *
+ * @param {Db} db - MongoDB database instance
+ * @param {string|ObjectId} id - User's MongoDB _id
+ * @param {string} role - New role ('user' or 'admin')
+ * @returns {Promise<Object|null>} Updated user or null
+ */
+async function updateUserRole(db, id, role) {
+  const objectId = typeof id === 'string' ? new ObjectId(id) : id;
+
+  const result = await db.collection(USER_COLLECTION).findOneAndUpdate(
+    { _id: objectId },
+    { $set: { role } },
+    { returnDocument: 'after' }
+  );
+
+  return result;
+}
+
+/**
+ * Check if a string is a valid MongoDB ObjectId
+ *
+ * @param {string} id - String to validate
+ * @returns {boolean} True if valid ObjectId format
+ */
+function isValidObjectId(id) {
+  if (typeof id !== 'string') return false;
+  if (!id || !id.trim()) return false;
+  return /^[0-9a-fA-F]{24}$/.test(id);
+}
+
 module.exports = {
   USER_COLLECTION,
   findOrCreateUser,
   findByGoogleId,
   findById,
   setUserActive,
-  createIndexes
+  createIndexes,
+  getPaginatedUsers,
+  updateUserRole,
+  isValidObjectId
 };