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/tasks/automation_tasks.py

@@ -0,0 +1,44 @@
+"""Celery task for asynchronous automation hook delivery with retry and backoff.
+
+Uses :class:`~app.tasks.retry_config.BaseTaskWithRetry` so failed deliveries
+are automatically retried with exponential backoff (default: 60 s, 300 s,
+900 s) and ±20 % jitter.
+"""
+
+import logging
+from typing import Any
+
+from app.celery_app import celery
+from app.tasks.retry_config import BaseTaskWithRetry
+from app.utils.webhook import deliver_webhook
+
+logger = logging.getLogger(__name__)
+
+
+@celery.task(base=BaseTaskWithRetry, bind=True, name="automation.deliver_hook")
+def deliver_automation_hook_task(self, url: str, payload: dict[str, Any], secret: str | None = None) -> dict[str, Any]:
+    """Deliver an automation hook payload to *url* with automatic retries.
+
+    Args:
+        url: Target webhook URL (provided by Zapier / Make.com).
+        payload: The flat Zapier-compatible payload.
+        secret: Optional shared secret for HMAC-SHA256 signing.
+
+    Returns:
+        A dict with ``status`` and ``url`` on success.
+
+    Raises:
+        RuntimeError: Re-raised to trigger Celery retry on delivery failure.
+    """
+    logger.info(
+        "Delivering automation hook to %s (attempt %d/%d)",
+        url,
+        self.request.retries + 1,
+        self.max_retries + 1,
+    )
+
+    success = deliver_webhook(url, payload, secret)
+    if success:
+        return {"status": "delivered", "url": url}
+
+    raise RuntimeError(f"Automation hook delivery to {url} failed")