feat(automation): add Zapier and Make.com integration

Add REST hooks subscription endpoints, incoming action endpoints, and
Zapier-compatible flat payload format for automation platform integration.

- AutomationHook model for webhook subscriptions
- POST /api/automation/hooks/subscribe and DELETE /hooks/{id}
- GET /api/automation/triggers/sample/{event} for Zapier field mapping
- POST /api/automation/actions/upload for incoming document uploads
- Celery task with retry for async hook delivery
- Integration with existing webhook dispatch flow
- 30 passing tests covering all new functionality

Co-authored-by: christianlouis <361235+christianlouis@users.noreply.github.com>

app/utils/settings_service.py

@@ -1595,6 +1595,18 @@ SETTING_METADATA = {
         "required": False,
         "restart_required": False,
     },
+    "automation_hooks_enabled": {
+        "category": "Feature Flags",
+        "description": (
+            "Enable Zapier / Make.com automation hook subscriptions and delivery. "
+            "When enabled, external automation platforms can subscribe to DocuElevate events "
+            "via the REST hooks protocol. Default: True."
+        ),
+        "type": "boolean",
+        "sensitive": False,
+        "required": False,
+        "restart_required": False,
+    },
     # Backup / Restore
     "backup_enabled": {
         "category": "Backup",