Core Notification Infrastructure & Snapshot Triggers

Implement the foundational infrastructure for the OWASP Nest notification system as part of #3227.

### Tasks
- [x] Implement `Subscription` and `Notification` models with GenericForeignKey support.
- [x] Configure Redis Stream (`owasp_notifications`) for event-driven messaging.
- [x] Implement a reliable Worker using Redis Consumer Groups and `XACK`.
- [x] Implement a robust Worker with PEL recovery (startup crash protection) and exponential backoff.
- [x] Add Dead Letter Queue (DLQ) with finite retries to prevent infinite loops.
- [x] Implement transition-aware Signal logic to prevent duplicate notification spam.
- [x] Add distributed locking to ensure safe, concurrent DLQ processing.
- [x] Support console-based email stimulation for local development.

This provides a "channel-agnostic" base that can easily be extended to other distribution channels