fix: deduplicate budget threshold alerts per period

[Calebstack]

Overview

This PR ensures budget threshold alerts fire at most once per budget per period. When a threshold is crossed, the alert time is recorded and duplicate alerts for the same period are suppressed until a new period begins.

Related Issue

Changes

🔔 Budget Alert Deduplication

• [MODIFY] app/services/token_usage.py

  • Record when a threshold alert is emitted and suppress duplicate alerts for the same budget within the current period.
  • Reset the recorded alert state when a new period starts so the threshold can fire again.

• [MODIFY] app/models/notification.py

  • Persist the alert timestamp/period marker needed to track and deduplicate threshold alerts.

• [MODIFY] tests/test_notifications.py

  • Add coverage for repeated threshold crossings within a period (deduplicated) and for re-alerting after a period reset.

Verification Results

ruff check .
black --check .
pytest

Acceptance Criteria	Status
Alerts are deduplicated per period	✅ Threshold alert recorded and duplicates suppressed within the period
Tests cover repeated crossings	✅ Added cases for repeated crossings and period reset
ruff check ., black --check . and pytest stay green	✅ Lint, format, and test suite pass

Closes #471

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