Add job monitoring, bulk import, channel update events, and notification export

[amanosiadnan-cmyk]

closes #492
closes #489
closes #481
closes #502

PR description

Summary

• Background job monitoring — JobMonitor records scheduled job status; failures are logged; APIs + docs for maintenance.
• Bulk notification import — Admins can import JSON/CSV batches; invalid rows are skipped; import summary is returned.
• Channel update events — Contract emits ChannelPreferenceUpdated on channel preference changes, with tests.
• Notification export — Search page Export button downloads filtered results as JSON/CSV; format documented.

Task 1 — Add Background Job Monitoring

• New listener/src/services/job-monitor.ts tracks running / completed / failed jobs.
• Wired into NotificationScheduler so each scheduled notification is monitored.
• Failed jobs logged via logger.error and retained in a failure list.
• Endpoints: GET /api/schedule/jobs, GET /api/schedule/jobs/failures
• Docs: docs/JOB_MONITORING.md (+ link from docs/MONITORING_INTEGRATION.md)
• Tests: job-monitor.test.ts

Task 2 — Support Bulk Notification Import

• New NotificationImportService accepts JSON arrays / { notifications: [] } or CSV.
• Invalid records skipped safely; valid rows scheduled through NotificationAPI.
• Endpoint: POST /api/notifications/import (honors X-API-Key when configured)
• Response summary: total, imported, skipped, importedIds, skippedRecords, format, durationMs
• Docs: docs/BULK_NOTIFICATION_IMPORT.md
• Tests: notification-import-service.test.ts

Task 3 — Emit Event for Channel Updates

• New ChannelPreferenceUpdated event in base/events.rs with recipient, channel, enabled, updated_at (+ category/priority topics).
• Emitted from set_channel_preference, set_preferences, and reset_preferences.
• Tests in preferences_test.rs assert emission and per-channel counts.

Task 4 — Add Notification Export Button

• Export controls on NotificationSearchPage (JSON/CSV).
• Uses current filters; paginates through all matching results before download.
• Helpers + format docs: dashboard/src/utils/notificationExport.ts, docs/NOTIFICATION_EXPORT_FORMAT.md
• Tests: notificationExport.test.ts

Test plan

• GET /api/schedule/jobs returns snapshot after scheduler activity
• Force a failed delivery → appears in /api/schedule/jobs/failures and logs
• POST /api/notifications/import with mixed valid/invalid JSON → summary shows skip + import
• Import CSV with missing recipient → row skipped, others imported
• set_channel_preference emits channel_preference_updated
• Notification Search: apply filters → Export JSON/CSV → file matches filters
• Confirm export format matches docs/NOTIFICATION_EXPORT_FORMAT.md

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