Fix data-layer bugs: bind_param fatals, ticket_id bindings, cache poisoning

- CustomFieldModel: assign ?? fallbacks to variables before bind_param
  (by-reference args cannot be ?? expressions; fatal on PHP 8.2, custom
  fields were uncreatable/uneditable)
- RecurringTicketModel::create: fix swapped bind type for schedule_type
  (enum bound as int coerced 'daily' to 0, breaking the cron)
- TicketModel/CommentModel: bind varchar ticket_id as string not int so
  the unique index is usable and leading-zero IDs match; ticket_watchers
  (int column) left as integer
- TicketModel::deleteTicket: delete from custom_field_values (real table)
  not the nonexistent ticket_custom_fields
- TicketModel search: honor literal '0'; never emit AGAINST('*') on
  all-special-char input (fall back to LIKE)
- TicketModel::updateTicket: disambiguate not-found vs no-op vs genuine
  optimistic-lock conflict on zero affected rows
- WorkflowModel: do not cache transitions/statuses on DB failure (a
  transient error no longer blocks all status changes for the TTL)
- DependencyModel: filter linked tickets by visibility (new optional user
  context params) to stop confidential metadata leaking via dependencies
- BulkOperationsModel: validate status/priority/assignee before mutating
- AuditLogModel: gate getClientIP forwarded headers on trusted proxies;
  add missing action/entity types so audit-log filters work
- WorkflowModel: add transitionRequiresComment() accessor for enforcement
- CommentModel: stop leaking raw DB errors to clients (log instead)

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

models/RecurringTicketModel.php

@@ -65,7 +65,7 @@ class RecurringTicketModel
 
         $stmt = $this->conn->prepare($sql);
         $stmt->bind_param(
-            'ssssiiisssii',
+            'ssssiissssii',
             $data['title_template'],
             $data['description_template'],
             $data['category'],