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

[Felamsy]

Overview

This PR adds offline test coverage for the billing alert and digest notification service. It exercises threshold evaluation, dedupe behavior, notification preferences, and the email fallback path, and makes the small supporting changes needed in the notification, email, and preference modules plus shared test fixtures to make those paths testable without network access.

Related Issue

Changes

🔔 Alert Service Tests

• [ADD] tests/test_alerts.py

  • Covers alert threshold evaluation for billing alerts.
  • Covers dedupe behavior so repeated alerts are not re-sent.
  • Covers notification preference handling.
  • Covers the email fallback path.
  • Kept fully offline (no live email/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 offline email fallback path.

• [MODIFY] app/models/notification_preference.py

  • Adjustments needed for preference handling used by the alert tests.

• [MODIFY] tests/conftest.py

  • Shared fixtures to keep the alert suite offline.

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	✅ No network calls; fixtures in tests/conftest.py keep tests offline
ruff check ., black --check . and pytest stay green	✅ All three commands pass

Closes #475

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