[Backend] Implement Notification Retry Queue

[Abd-Standard]

Description

Introduce a retry mechanism for notifications that fail due to temporary service interruptions.

Tasks
Create retry queue service.
Configure retry intervals.
Track retry attempts.
Log final failures.

Acceptance Criteria
Failed notifications are retried automatically.
Retry limits are respected.
Logs contain retry history.

[naninu123]

Hi, interested in contributing to this.

I have experience across Stellar OSS bounties through GrantFox with merged PRs in Core-Foundry/Notify-Chain, Flux-DeFi/LiquidFlow, StellarLend, and aid-linkk. I adapt quickly to different codebases.

Approach:

1. Study the issue and surrounding code
2. Implement focused changes following repo conventions
3. Test and submit a clean PR

Available to start right away!