chore: add type annotations to notifications

[nijel]

Also fixes some of the unhandled conditions.

[codecov]

Codecov Report

Attention: Patch coverage is 70.00000% with 6 lines in your changes missing coverage. Please review.

Project coverage is 90.99%. Comparing base (ca82ccc) to head (cb51115).
Report is 3 commits behind head on main.

Files with missing lines	Patch %	Lines
weblate/accounts/notifications.py	68.42%	2 Missing and 4 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #14058      +/-   ##
==========================================
- Coverage   91.00%   90.99%   -0.01%     
==========================================
  Files         608      608              
  Lines       63648    63654       +6     
  Branches     6584     6587       +3     
==========================================
+ Hits        57923    57924       +1     
- Misses       4031     4033       +2     
- Partials     1694     1697       +3     

Files with missing lines	Coverage Δ
weblate/auth/models.py	92.03% <100.00%> (ø)
weblate/accounts/notifications.py	94.07% <68.42%> (-0.93%)	⬇️