refactor: optimize data_for_meetings_overview

[jennifer-richards]

Addresses #11279.

Refactors the data_for_meetings_overview() utility method, speeding it up by about 3x (from about 160 ms to 66 ms in my local environment).

Puts a one-year look-back limit on the interim_announce and interim_pending views. This speeds the calls in these views by a further 10x. E.g., for the query in interim_announce:

>>> timeit.timeit(lambda: data_for_meetings_overview(Meeting.objects.filter(type="interim"), interim_status="scheda"), number=30) / 30
0.066658752799655
>>> timeit.timeit(lambda: data_for_meetings_overview(Meeting.objects.filter(type="interim", date__gte="2025-08-27"), interim_status="scheda"), number=30) / 30
0.007071522233309225

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 88.73%. Comparing base (88cb3bf) to head (38fcad3).
⚠️ Report is 3 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main   #11657   +/-   ##
=======================================
  Coverage   88.73%   88.73%           
=======================================
  Files         337      337           
  Lines       45313    45316    +3     
=======================================
+ Hits        40209    40212    +3     
  Misses       5104     5104           

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