Fix API correctness: external API stub/collision, recurring dates, CSV, audit

- create_ticket_api.php: remove the wrong CREATE TABLE stub that broke a
  fresh DB; generate collision-safe ticket_ids so a genuine id collision
  isn't misreported as a duplicate and a hw alert dropped; stop leaking
  raw DB errors; correct a reopen comment that falsely claimed refreshed
  sensor data
- manage_recurring.php: fix next-run so create/edit no longer skips the
  current period (monthly day-of-month this month, daily today if time
  not passed, correct ISO weekday, month-length clamp); only recompute
  on schedule changes to avoid double-fire
- export_tickets.php, audit_log.php: neutralize CSV formula injection
- revoke_api_key.php, generate_api_key.php: correct HTTP status codes and
  stop the catch clobbering specific 4xx codes
- health.php: stop leaking PHP version / extension names / paths to
  unauthenticated callers
- watch_ticket.php: define $data before use
- manage_templates/recurring/custom_fields: add audit logging for CRUD;
  add recurring_ticket + custom_field to the audit entity whitelist

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

models/AuditLogModel.php

@@ -27,7 +27,7 @@ class AuditLogModel
     private const VALID_ENTITY_TYPES = [
         'ticket', 'comment', 'user', 'api_key', 'security',
         'template', 'attachment', 'ticket_attachments', 'group',
-        'dependency', 'workflow_transition'
+        'dependency', 'workflow_transition', 'recurring_ticket', 'custom_field'
     ];
 
     public function __construct($conn)