feat(notifications): implement tenant-scoped broadcast notifications

[KilianTrunk]

No description provided.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 73.41%. Comparing base (391bf0f) to head (a6f15e0).
⚠️ Report is 2 commits behind head on main.

Additional details and impacted files

@@             Coverage Diff              @@
##               main      #23      +/-   ##
============================================
+ Coverage     73.28%   73.41%   +0.12%     
- Complexity      290      291       +1     
============================================
  Files            54       54              
  Lines          1868     1877       +9     
============================================
+ Hits           1369     1378       +9     
  Misses          499      499              

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[SlimDeluxe]

Looks ok, but let's wait for the fix in https://github.com/DataLinx/singa.si/pull/7