[508] Document notification processing flow

[NanaKhadija1980j]

Summary

• Documented the end-to-end notification processing lifecycle.
• Added architecture and processing flow diagrams.
• Documented failure and retry behavior (including dead-letter queue for scheduled notifications).
• Added troubleshooting guidance based on the actual implementation.

Closes #508

Test plan

• Traced processing stages against source code (EventSubscriber, queues, Discord delivery, schedulers, DLQ).
• Verified diagrams against implementation.
• Verified internal documentation links.
• Checked Markdown/Mermaid syntax visually.

Validation

• Stages limited to what exists in listener/src (no invented providers or buses).
• Documented real-time vs scheduled pipelines, async boundaries, persistence vs transient state, and idempotency layers.
• Included dead_letter_queue behavior introduced on main.
• No markdownlint tooling is configured in this repository; validation was manual path/link/source tracing.
• Documentation reviewed against source by the authoring agent (not a human maintainer review).
• No application logic changes.

Made with Cursor

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