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/BulkOperationsModel.php

@@ -77,6 +77,15 @@ class BulkOperationsModel
 
         $ticketIds = explode(',', $operation['ticket_ids']);
         $parameters = $operation['parameters'] ? json_decode($operation['parameters'], true) : [];
+
+        // Validate operation parameters up front so invalid values (out-of-range
+        // priority, unknown status, nonexistent assignee) are rejected cleanly
+        // instead of corrupting tickets or throwing mid-transaction.
+        $paramError = $this->validateOperationParameters($operation['operation_type'], is_array($parameters) ? $parameters : []);
+        if ($paramError !== null) {
+            return ['processed' => 0, 'failed' => count($ticketIds), 'error' => $paramError];
+        }
+
         $processed = 0;
         $failed = 0;
         $errors = [];
@@ -297,6 +306,63 @@ class BulkOperationsModel
         return $result;
     }
 
+    /**
+     * Validate the parameters for a bulk operation before any ticket is mutated.
+     *
+     * @return string|null Error message, or null if the parameters are valid
+     */
+    private function validateOperationParameters(string $type, array $parameters): ?string
+    {
+        switch ($type) {
+            case 'bulk_priority':
+                if (!isset($parameters['priority'])) {
+                    return 'Missing priority parameter';
+                }
+                $priority = $parameters['priority'];
+                // tickets.priority has a CHECK constraint (between 1 and 6).
+                if (!is_numeric($priority) || (int)$priority < 1 || (int)$priority > 6) {
+                    return 'Invalid priority: must be between 1 and 6';
+                }
+                break;
+
+            case 'bulk_status':
+                if (!isset($parameters['status'])) {
+                    return 'Missing status parameter';
+                }
+                $validStatuses = $GLOBALS['config']['TICKET_STATUSES']
+                    ?? ['Open', 'Pending', 'In Progress', 'Closed'];
+                if (!in_array($parameters['status'], $validStatuses, true)) {
+                    return 'Invalid status value';
+                }
+                break;
+
+            case 'bulk_assign':
+                if (!isset($parameters['assigned_to'])) {
+                    return 'Missing assigned_to parameter';
+                }
+                $assignedTo = $parameters['assigned_to'];
+                if (!is_numeric($assignedTo) || (int)$assignedTo <= 0 || !$this->userExists((int)$assignedTo)) {
+                    return 'Invalid assigned_to: user does not exist';
+                }
+                break;
+        }
+
+        return null;
+    }
+
+    /**
+     * Check whether a user ID exists.
+     */
+    private function userExists(int $userId): bool
+    {
+        $stmt = $this->conn->prepare("SELECT 1 FROM users WHERE user_id = ? LIMIT 1");
+        $stmt->bind_param("i", $userId);
+        $stmt->execute();
+        $exists = $stmt->get_result()->num_rows > 0;
+        $stmt->close();
+        return $exists;
+    }
+
     /**
      * Get bulk operation by ID
      *