fix(task_sync): defend against silent sync_id integrity violations

The cross-instance sync silently drops tasks when x_fc_tech_sync_id is
missing on the technician, and silently collapses duplicates via dict
comprehension. Both make sync break in ways that are invisible until
someone notices a missing task on the other instance.

- _get_remote_tech_map / _get_local_syncid_to_uid: warn on duplicates
- _push_tasks_to_remote: info-log when a task is skipped because the
  tech has no sync_id or no remote counterpart
- res.users onchange: warn in the form when entering a sync_id that
  is already used by another active field staff

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

fusion_tasks/__manifest__.py

@@ -4,7 +4,7 @@
 
 {
     'name': 'Fusion Tasks',
-    'version': '19.0.1.0.0',
+    'version': '19.0.1.1.0',
     'category': 'Services/Field Service',
     'summary': 'Technician scheduling, route planning, GPS tracking, and cross-instance sync.',
     'author': 'Nexa Systems Inc.',