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

.htaccess

@@ -132,6 +132,8 @@ RewriteRule ^update_album$ src/processors/update_album.php [L]
 RewriteRule ^delete_album$ src/processors/delete_album.php [L]
 RewriteRule ^delete_photo$ src/processors/delete_photo.php [L]
 RewriteRule ^get_album_photos$ src/processors/get_album_photos.php [L]
+RewriteRule ^link_membership_user$ src/processors/link_membership_user.php [L]
+RewriteRule ^unlink_membership_user$ src/processors/unlink_membership_user.php [L]
 
 </IfModule>