feat: track and expose blockchain polling telemetry

[susanyusuf]

Summary

Adds in-memory telemetry for blockchain polling cycles in the listener and exposes it through the existing health/diagnostic interface.

Changes

• New \PollingMetrics\ collector (\listener/src/services/polling-metrics.ts) that records each poll cycle's duration and outcome (success/failure), with running totals, success rate, and recent durations.
• \EventSubscriber\ now records every poll cycle into the collector — successful and failed cycles are recorded separately.
• \NotificationHealthMonitor\ health report now includes a \polling\ section (via \GET /api/notifications/health) exposing the last successful/failed poll timestamp, last poll duration, outcome, and totals.

Issues closed

Closes #618
Closes #619
Closes #620

PR targets upstream \main.

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