feat: implement comprehensive admin functionality

- Add admin role system with role and isAdmin fields to users table
- Create admin_actions audit log table for tracking all admin operations
- Implement admin CLI tool for user management (create, promote, demote, list, check)
- Add admin authentication with role-based access control
- Create admin service layer with system statistics and user management
- Implement user impersonation system with proper security checks
- Add admin API endpoints for user management and system statistics
- Create admin dashboard UI with overview, users, and action logs
- Fix admin stats endpoint and user deletion with proper foreign key handling
- Add admin link to navigation bar for admin users

Database:
- Add role and isAdmin columns to users table
- Create admin_actions table for audit trail
- Migration script: add-admin-functionality.js

CLI:
- src/backend/scripts/admin-cli.js - Admin user management tool

Backend:
- src/backend/services/admin.service.js - Admin business logic
- Updated auth.service.js with admin helper functions
- Enhanced index.js with admin routes and middleware
- Export sqlite connection from config for raw SQL operations

Frontend:
- src/frontend/src/routes/admin/+page.svelte - Admin dashboard
- Updated api.js with adminAPI functions
- Added Admin link to navigation bar

Security:
- Admin-only endpoints with role verification
- Audit logging for all admin actions
- Impersonation with 1-hour token expiration
- Foreign key constraint handling for user deletion
- Cannot delete self or other admins
- Last admin protection

src/backend/services/auth.service.js

@@ -142,3 +142,102 @@ export async function updateDCLCredentials(userId, dclApiKey) {
     })
     .where(eq(users.id, userId));
 }
+
+/**
+ * Check if user is admin
+ * @param {number} userId - User ID
+ * @returns {Promise<boolean>} True if user is admin
+ */
+export async function isAdmin(userId) {
+  const [user] = await db
+    .select({ isAdmin: users.isAdmin })
+    .from(users)
+    .where(eq(users.id, userId))
+    .limit(1);
+
+  return user?.isAdmin === true || user?.isAdmin === 1;
+}
+
+/**
+ * Get all admin users
+ * @returns {Promise<Array>} Array of admin users (without passwords)
+ */
+export async function getAdminUsers() {
+  const adminUsers = await db
+    .select({
+      id: users.id,
+      email: users.email,
+      callsign: users.callsign,
+      role: users.role,
+      isAdmin: users.isAdmin,
+      createdAt: users.createdAt,
+    })
+    .from(users)
+    .where(eq(users.isAdmin, 1));
+
+  return adminUsers;
+}
+
+/**
+ * Update user role
+ * @param {number} userId - User ID
+ * @param {string} role - New role ('user' or 'admin')
+ * @param {boolean} isAdmin - Admin flag
+ * @returns {Promise<void>}
+ */
+export async function updateUserRole(userId, role, isAdmin) {
+  await db
+    .update(users)
+    .set({
+      role,
+      isAdmin: isAdmin ? 1 : 0,
+      updatedAt: new Date(),
+    })
+    .where(eq(users.id, userId));
+}
+
+/**
+ * Get all users (for admin use)
+ * @returns {Promise<Array>} Array of all users (without passwords)
+ */
+export async function getAllUsers() {
+  const allUsers = await db
+    .select({
+      id: users.id,
+      email: users.email,
+      callsign: users.callsign,
+      role: users.role,
+      isAdmin: users.isAdmin,
+      createdAt: users.createdAt,
+      updatedAt: users.updatedAt,
+    })
+    .from(users)
+    .orderBy(users.createdAt);
+
+  return allUsers;
+}
+
+/**
+ * Get user by ID (for admin use)
+ * @param {number} userId - User ID
+ * @returns {Promise<Object|null>} Full user object (without password) or null
+ */
+export async function getUserByIdFull(userId) {
+  const [user] = await db
+    .select({
+      id: users.id,
+      email: users.email,
+      callsign: users.callsign,
+      role: users.role,
+      isAdmin: users.isAdmin,
+      lotwUsername: users.lotwUsername,
+      dclApiKey: users.dclApiKey,
+      createdAt: users.createdAt,
+      updatedAt: users.updatedAt,
+    })
+    .from(users)
+    .where(eq(users.id, userId))
+    .limit(1);
+
+  return user || null;
+}