Write integration tests for cron trigger lifecycle

Organize tests in a file named `cron_trigger_integration_test.dart` under the `integration_test/` directory. Use a `CronTestHelpers` utility class to encapsulate common setup patterns (seeding an assignment, inserting an activity, reading log rows). For the advisory lock concurrency test, insert a deliberate 2-second delay into the Edge Function via a test-mode flag to make the race condition reliably observable — remove this flag from production builds. Use Supabase's `supabase_flutter` client with the test project's URL and service role key, loaded from environment variables in test setup.

Consider using Supabase's branching feature (if available) to create a clean database branch per test run for maximum isolation.

This task IS the testing deliverable. Each of the 8 acceptance criteria test cases must be implemented as a named test in flutter_test or a Deno test file (depending on where the Edge Function logic is tested). Use `setUp` and `tearDown` hooks to isolate test state. For the concurrency test, use Dart's `Future.wait` to invoke the cron function twice simultaneously.

For the activity reset test, use the Supabase test client to insert an activity row directly and then assert the assignments table state. Mock the alerting webhook using a local HTTP server spun up in the test (e.g., using Dart's `HttpServer`). Assert both the HTTP call was received AND the payload matches the expected structure.