Add comprehensive e2e tests for user-level webhook CRUD operations
and update existing project webhook tests to use LoadFixtures() for
cleanup instead of manual DELETE queries.
New pkg/e2etests/ package runs with the real Watermill event system to
verify the full async pipeline: web handler -> DB -> event dispatch ->
Watermill -> listener -> side effect.
First test (TestTaskUpdateWebhookE2E) updates a task and asserts that
a webhook HTTP POST arrives at a test server.
