test: add offline alert service tests for billing alerts and digests

[Felamsy]

Overview

This PR adds offline test coverage for billing alerts and digests, covering thresholds, dedupe, notification preferences, and email fallback. Supporting changes in the notification and email services and the notification preference model make the alert behavior testable without network access.

Related Issue

Changes

🔔 Alert Service Tests

• [ADD] tests/test_alerts.py

  • Covers threshold triggering for billing alerts.
  • Covers dedupe behavior so repeated alerts are not re-sent.
  • Covers notification preference handling.
  • Covers email fallback path.
  • Runs fully offline (no live email or network calls).

• [MODIFY] app/services/notifications.py

  • Adjustments needed to support threshold, dedupe, and preference behavior under test.

• [MODIFY] app/services/email.py

  • Adjustments needed to support the email fallback path offline.

• [MODIFY] app/models/notification_preference.py

  • Adjustments needed to support preference handling in alert tests.

Verification Results

ruff check .
black --check .
pytest

Acceptance Criteria	Status
Alert behavior is fully covered	✅ Thresholds, dedupe, preferences, and email fallback covered in tests/test_alerts.py
The suite runs offline	✅ Tests avoid live network/email calls
ruff check ., black --check . and pytest stay green	✅ Lint, format, and test suite pass

Closes #475

[Felamsy]

this PR is ready for another review. I resolved the merge conflicts with the main branch (pushed 70c16d9).
Files changed: app/services/email.py, tests/test_alerts.py