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/config.py

@@ -617,6 +617,12 @@ class Settings(BaseSettings):
         description="Enable webhook delivery for document events",
     )
 
+    # Automation hooks (Zapier / Make.com)
+    automation_hooks_enabled: bool = Field(
+        default=True,
+        description="Enable Zapier / Make.com automation hook subscriptions and delivery",
+    )
+
     # ── Backup / restore settings ──────────────────────────────────────────────
     backup_enabled: bool = Field(
         default=True,