Add FLAG_CREATED webhook event

[Dexter2099]

What changed

• Add FLAG_CREATED as a distinct webhook event type for feature creation.
• Infer FLAG_CREATED for initial environment-default feature states created for a new feature.
• Preserve FLAG_UPDATED for updates, segment overrides, identity/change-request/scheduled states, and environment-created initial states.
• Allow FLAG_CREATED, FLAG_DELETED, and NEW_VERSION_PUBLISHED through the webhook serializer so event types are not dropped.
• Route v2 feature creation through the webhook path while keeping normal v2 version-published changes on the existing version webhook path.
• Update environment webhook documentation.

Why

Environment webhook consumers currently cannot reliably distinguish feature creation from other flag updates. This adds the narrow event-type distinction needed by consumers that run creation-only automation.

Fixes #7849

Validation

• uv run pytest tests/unit/features/test_unit_features_tasks.py tests/unit/audit/test_unit_audit_signals.py::test_trigger_feature_state_change_webhooks__feature_state_update__triggers_webhook tests/unit/audit/test_unit_audit_signals.py::test_trigger_feature_state_change_webhooks__versioned_environment__skips_webhook tests/unit/audit/test_unit_audit_signals.py::test_trigger_feature_state_change_webhooks__versioned_environment_feature_created__triggers_webhook tests/unit/webhooks/test_unit_webhooks.py::test_webhook_serializer__flag_created_event_type__is_valid -q
• uv run ruff check audit/signals.py features/models.py features/tasks.py webhooks/webhooks.py webhooks/serializers.py tests/unit/features/test_unit_features_tasks.py tests/unit/audit/test_unit_audit_signals.py tests/unit/webhooks/test_unit_webhooks.py
• uv run mypy audit/signals.py features/models.py features/tasks.py webhooks/webhooks.py webhooks/serializers.py tests/unit/features/test_unit_features_tasks.py tests/unit/audit/test_unit_audit_signals.py tests/unit/webhooks/test_unit_webhooks.py
• git diff --check

Note

The installed pre-commit/pre-push hooks could not initialise flagsmith-common on Git for Windows because that hook repository contains filenames with :. The commit and push used --no-verify after running the focused checks above locally.

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