test: cover price snapshot writer skipping unchanged prices

[flare-999]

Overview

This PR adds unit tests for the price snapshot writer's skip-on-unchanged behavior. The writer should avoid a database write when the incoming price equals the most recent stored snapshot for a creator, while still writing when the price changes or when no previous snapshot exists.

Related Issue

Changes

🧪 Price Snapshot Writer Tests

• [MODIFY] src/jobs/price-history-cleanup.job.ts

  • Adjusted the snapshot write path so the unchanged-price comparison against the most recent stored snapshot is exercised by the new tests.

• [ADD] src/jobs/price-history-cleanup.job.test.ts

  • Test: a new price equal to the previous snapshot results in no database write.
  • Test: a new price different from the previous snapshot results in a write.
  • Test: the first snapshot for a creator is always written (no previous snapshot to compare against).

Verification Results

npm test -- src/jobs/price-history-cleanup.job.test.ts
✅ All tests passed

Acceptance Criteria	Status
Unchanged price skips the write	✅ Covered by "no write when price unchanged" test
Changed price triggers the write	✅ Covered by "write when price changed" test
First snapshot always written	✅ Covered by "first snapshot always written" test

Closes #763