[EPIC] Test foundation

The #597 service-extraction EPIC (#589–#599) is now **merged**, and the test suite has grown well beyond the original "one unit test": `test/` now holds ~51 passing tests across `services_test.dart`, `view_models_test.dart`, `app_bloc_test.dart`, `cams_app_test.dart`, plus `heart_rate_data_model_test.dart`.

This EPIC now tracks the **remaining gaps** on top of that foundation so the other EPICs can land safely:
- Formalize per-service coverage assertions and shared fixtures (#644)
- Stream-driven tests for the card view-models, once the leak EPIC (#613) fixes single-subscription behavior (#645)
- Widget tests for the router redirect + consent gate — currently untested (#646)
- A CI coverage floor so the suite can only grow (#647)

**Definition of done:** every refactored view-model and the two critical flows (login redirect, consent gate) have tests running in CI with a ratchet-only coverage floor.

_Note: original body referenced branch `chore/cams-2.x-upgrade` @ 560a181; the service refactor has since merged to the mainline._