Add instance-level webhooks

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Title

Add instance-level webhooks and create:user event

Body

Motivation and context

Enterprise customers need a webhook when a user is created. CVAT previously only supported project- and organization-scoped webhooks, so there was no place to subscribe to instance-wide events such as user creation.

Adds WebhookTypeChoice.INSTANCE (no project/org), configured only via Django admin. Instance event catalog: create:user, create:organization, delete:organization. create:user is dispatched from Django post_save on User (skips raw and is_superuser). Public REST continues to reject/exclude instance webhooks.

Breaking: GET /api/webhooks/events now requires type (project | organization); the previous default all catalog is removed. Later user.registered / user.verified / first-login events are deferred.

How has this been tested?

Checklist

• I submit my changes into the develop branch
• I have created a changelog fragment
• I have updated the documentation accordingly
• I have added tests to cover my changes
• I have linked related issues (see GitHub docs)

License

• I submit my code changes under the same MIT License that covers the project.
  Feel free to contact the maintainers if that's a concern.