Introduce notification deduplication service

[eischideraa-unn]

Summary

Introduces a configurable notification deduplication service for the listener backend to prevent identical notifications from being processed repeatedly within a time window.

Changes

• Added TTL-based notification deduplication using generated fingerprints.
• Added configurable deduplication settings:
  • NOTIFICATION_DEDUPLICATION_WINDOW_MS
  • NOTIFICATION_DEDUPLICATION_MAX_SIZE
• Added deduplication metrics for:
  • accepted requests
  • skipped duplicates
  • evicted cache entries
  • expired cache entries
  • current cache size
  • configured deduplication window
• Updated Discord notification handling so duplicate skips are treated as handled, not retryable failures.
• Added test coverage for duplicate skipping, TTL expiry, metrics, and config loading.

Acceptance Criteria

• Duplicate notifications are ignored.
• Legitimate new requests continue processing.
• Duplicate notifications are allowed again after the configured deduplication window expires.
• Metrics accurately report skipped duplicates and cache activity.

closes #66

Verification

• npm test -- --runInBand
  • 10 test suites passed
  • 109 tests passed
• npm run build
  • TypeScript build passed