fix: add retries and provider failover to notification delivery

[DevALVIN-24]

closes #1210

Description:
This pull request addresses the transient provider outage issue that previously led to silently dropped user-facing notifications.

Changes:

• Retry Mechanism: Implemented inline exponential backoff (e.g. 1s, 2s, 4s, etc.) up to 3 max retries to protect against transient provider failures without dropping logs.
• Permanent Failure Recording: Persists error stack and failure reasons on the log's metadata.failureReason on exhausting retry limits or striking a permanent error for queryability via getDeliveryLogsForUser.
• Provider & Channel Failover:
  • Added primary to secondary provider failover for email delivery.
  • Built channel fallback support that auto-routes CRITICAL severity notifications from Push to Email, or Email to SMS, ensuring vital deliveries arrive despite channel blackouts.
• Prometheus Metrics: Integrated existing @Global MetricsService to export counts for delivery attempts, successes, permanent failures, and failovers.
• Tracked Issues: Updated all raw markers to sequentially tracked codebase pointers (#1021 through #1025).

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[Cedarich]

@DevALVIN-24 kindly fix CI

[Cedarich]

Closing this as part of the Wave 8 → Wave 9 transition.

The board is being reset so Wave 9 issues can be opened against a clean backlog. This PR has been open 26 days without landing. All issues are being unassigned at the same time — the issues themselves stay open.

This is not a rejection of your work. If you want to carry it forward into Wave 9:

1. Reopen this PR, or comment here and we will reopen it.
2. Comment on the linked issue to have it reassigned to you.

Thanks for contributing to LumenPulse.