feat: [five-c] AboutCode app for alerts and notifications

[tdruez]

Issues

Issue: #412

Changes

django-rest-hooks removal

• Removed the django-rest-hooks dependency and all related configuration (HOOK_FINDER, HOOK_DELIVERER, HOOK_EVENTS, HOOK_ENV settings).
• Removed the legacy Webhook model and replaced it with WebhookSubscription and WebhookDelivery.
• Existing webhook records are migrated to the new model via a data migration.

aboutcode/notifications module

• Added a new aboutcode/notifications module with AbstractWebhookSubscription, AbstractWebhookDelivery, and WebhookSubscriptionQuerySetMixin as reusable abstract bases.
• The delivery logic now lives in the abstract, making it available to other projects (e.g. ScanCode.io) without duplication.

Webhook models

• WebhookSubscription replaces the old Webhook model. The target field is renamed to target_url, and extra_payload/extra_headers are retained.
• WebhookDelivery records each delivery attempt with the payload sent, HTTP status code, response text, and any errors.
• Delivery is now fully async via RQ. Each fire_webhooks() call enqueues a deliver_webhook_task per matching subscription.
• WEBHOOK_EVENTS is now defined in notification/models.py rather than settings.
• The HOOK_ENV setting is renamed to DEJACODE_WEBHOOK_ENV, with backward compatibility for deployments still using the old name.

Admin

• Added a read-only inline showing the 10 most recent deliveries when editing a subscription, with sent_date, response_status_code, and delivery_error.

Screens