feat: implement membership linking system for couples and family members

- Created membership_links table to associate secondary users with primary memberships
- Created membership_permissions table for granular permission control
- Added linkSecondaryUserToMembership() function to create links with validation
- Added getUserMembershipLink() to check access via secondary links
- Added getLinkedSecondaryUsers() to list all secondary users for a primary member
- Added unlinkSecondaryUser() to remove links
- Updated getUserMemberStatus() to check both direct and linked memberships
- Created link_membership_user processor to handle linking via API
- Created unlink_membership_user processor to handle unlinking via API
- Added .htaccess routes for linking endpoints
- Grants default permissions: access_member_areas, member_pricing, book_campsites, book_courses, book_trips
- Includes transaction safety with rollback on errors
- Includes comprehensive documentation with usage examples
- Validates primary user has active membership before allowing links
- Prevents duplicate links and self-linking

src/processors/link_membership_user.php

@@ -0,0 +1,57 @@
+<?php
+$rootPath = dirname(dirname(__DIR__));
+require_once($rootPath . '/src/config/env.php');
+require_once($rootPath . '/src/config/session.php');
+require_once($rootPath . '/src/config/connection.php');
+require_once($rootPath . '/src/config/functions.php');
+
+header('Content-Type: application/json');
+
+if (!isset($_SESSION['user_id']) || $_SERVER['REQUEST_METHOD'] !== 'POST') {
+    http_response_code(403);
+    exit(json_encode(['success' => false, 'message' => 'Forbidden']));
+}
+
+// Validate CSRF token
+if (!isset($_POST['csrf_token']) || !validateCSRFToken($_POST['csrf_token'])) {
+    http_response_code(400);
+    exit(json_encode(['success' => false, 'message' => 'Invalid request']));
+}
+
+$primary_user_id = intval($_SESSION['user_id']);
+$secondary_email = trim($_POST['secondary_email'] ?? '');
+$relationship = trim($_POST['relationship'] ?? 'spouse');
+
+if (empty($secondary_email)) {
+    http_response_code(400);
+    exit(json_encode(['success' => false, 'message' => 'Secondary user email is required']));
+}
+
+// Get the secondary user by email
+$conn = openDatabaseConnection();
+$userQuery = $conn->prepare("SELECT user_id FROM users WHERE email = ?");
+$userQuery->bind_param("s", $secondary_email);
+$userQuery->execute();
+$userResult = $userQuery->get_result();
+$userQuery->close();
+
+if ($userResult->num_rows === 0) {
+    $conn->close();
+    http_response_code(404);
+    exit(json_encode(['success' => false, 'message' => 'User with that email not found']));
+}
+
+$user = $userResult->fetch_assoc();
+$secondary_user_id = $user['user_id'];
+$conn->close();
+
+// Use the linking function from functions.php
+$result = linkSecondaryUserToMembership($primary_user_id, $secondary_user_id, $relationship);
+
+http_response_code($result['success'] ? 200 : 400);
+echo json_encode([
+    'success' => $result['success'],
+    'message' => $result['message'],
+    'link_id' => $result['link_id'] ?? null
+]);
+?>