Refactor notification system with service approach

[fiftin]

Add a new service-based notification system with a common interface, using token/channel parameters and a new notifications config, while preserving old URL-based methods for backward compatibility.

This change introduces a more secure and modular way to handle notifications by moving away from insecure URL parameters and providing a structured configuration, ensuring existing setups continue to function without modification.

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[cursor]

Cursor Agent can help with this pull request. Just @cursor in comments and I'll start working on changes in this branch.
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