feat: Add sender-based routing support with backward compatibility (Fixes #145)

[EINDEX]

Summary

Adds support for routing notifications based on sender email address in addition to recipient address.

Backward Compatible: Users do not need to upgrade the config for this update.

Resolves #145

Changes

• Enhanced SimpleRouter to match on both sender and receiver patterns using fnmatch
• Supports two configuration formats:
  • Direct: sender -> config (implicit *@* receiver)
  • Nested: sender -> receiver -> config

Example

Sender-based routing:

configs:
  unifi@domain.com:
    urls:
      - pushover://...
  uisp@domain.com:
    urls:
      - discord://...

Fine-grained control:

configs:
  monitoring@*:
    alerts@mycompany.com:
      urls:
        - slack://...## Testing

• ✅ Added tests for direct and nested configurations
• ✅ Pattern matching with wildcards
• ✅ All existing tests pass (backward compatible)

Note

This integrates sender routing directly into the core router, unlike my import_code gist approach.
This provides better maintainability and doesn't require separate config files.