feat: health monitor, schema versioning, access logs, event reference guide

[Darkdruce]

Summary

Implements four issues in a single PR.

Closes #311 — [Backend] Notification Health Monitor

• New NotificationHealthMonitor service (listener/src/services/notification-health-monitor.ts)
• Monitors queue depth, stalled-job detection, worker availability, and event registry lag
• Generates health reports on a configurable interval
• Exposes reports at GET /api/notifications/health
• Wired into index.ts with graceful shutdown

Closes #309 — [Contracts] Notification Version Tracking

• Added SchemaVersionSet event to base/events.rs
• Added SchemaVersion storage key, set_schema_version, get_schema_version, is_version_supported in autoshare_logic.rs
• Exposed all three as public contract methods in lib.rs
• Unsupported versions are rejected with Error::InvalidInput; only admin can set version
• Tests: tests/schema_version_test.rs

Closes #312 — [Contracts] Notification Access Logs

• Added NotificationAccessed event to base/events.rs
• Added record_notification_access in autoshare_logic.rs
• Exposed as public contract method in lib.rs
• Tests: tests/access_log_test.rs

Closes #274 — [Docs] Smart Contract Event Reference Guide

• New CONTRACT_EVENT_REFERENCE.md documenting all emitted events, shared types, parameters, examples, and indexer/listener recommendations
• Linked from README.md

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