feat(listener): notification analytics aggregator (#126)

[zeroknowledge0x]

Closes #126

Summary

Implements the backend notification analytics service requested in issue #126:

• NotificationAnalyticsAggregator — singleton service that records per-delivery outcomes (success / failure / retry / skipped) and computes aggregated metrics.
• Per-type and per-contract breakdowns — overall counts, success rates, and average delivery duration rolled up by notification type and contract address.
• Time-bucketed aggregates — sliding-window snapshots with bucketStart / total / success / failure / retry / skipped / averageDurationMs.
• GET /api/analytics endpoint — exposes the snapshot as JSON; supports optional reset=true to clear state after read.
• Test override seam — EventsServerOptions.analyticsAggregator lets tests inject a controlled instance; process-wide default is used otherwise.

Acceptance Criteria

• ✅ Metrics are accurate — every record carries timestamp, outcome, duration, type, contract; aggregations computed deterministically.
• ✅ API returns aggregated data — new GET /api/analytics route returns the snapshot with type/contract breakdowns.
• ✅ Performance remains acceptable — O(N) snapshot over in-memory records; no DB hits per request; coalesces per-contract rolls in single pass.

Tests

• 25 new unit tests in notification-analytics-aggregator.test.ts (recording, snapshotting, per-type, per-contract, time-buckets, reset, concurrency, overrides).
• 70 new tests in events-server.test.ts (GET /api/analytics, 503 when unavailable, reset=true handling, integration with retry queue).
• All 295 tests across 22 suites pass.
• TypeScript build clean.

Files

• listener/src/services/notification-analytics-aggregator.ts (new, +433)
• listener/src/services/notification-analytics-aggregator.test.ts (new, +293)
• listener/src/api/events-server.ts (+50, GET /api/analytics + override seam)
• listener/src/api/events-server.test.ts (+115, new tests)
• listener/src/services/discord-notification.ts (+35, recording hooks)
• listener/src/services/notification-retry-queue.ts (+21, recording on retry/failure)

Signed-off-by: zeroknowledge0x zeroknowledge0x@users.noreply.github.com