feat(webhooks): add webhook support for external integrations

Add WebhookConfig model, CRUD API endpoints, HMAC-SHA256 signed delivery,
and Celery-based async dispatch with retry/backoff for document events
(document.uploaded, document.processed, document.failed).

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

app/config.py

@@ -274,6 +274,12 @@ class Settings(BaseSettings):
         description="Send notifications when files are successfully processed",
     )
 
+    # Webhook settings
+    webhook_enabled: bool = Field(
+        default=True,
+        description="Enable webhook delivery for document events",
+    )
+
     # File upload size limits (for security - see SECURITY_AUDIT.md)
     max_upload_size: int = Field(
         default=1073741824,  # 1GB in bytes (1024 * 1024 * 1024)