feat: matrix testing for python 3.14

[matmair]

Adds a CI step and 3.14 optimized requirement export for python 3.14 to ensure upstream compatibility (3.14 is the first free-thread version).

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 88.18%. Comparing base (6311f75) to head (e68e3b6).
⚠️ Report is 323 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master   #10904      +/-   ##
==========================================
+ Coverage   88.17%   88.18%   +0.01%     
==========================================
  Files        1290     1290              
  Lines       58223    58223              
  Branches     1968     1968              
==========================================
+ Hits        51336    51342       +6     
+ Misses       6396     6390       -6     
  Partials      491      491              

Flag	Coverage Δ
backend	89.47% <ø> (+0.01%)	⬆️
migrations	42.41% <ø> (+<0.01%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Components	Coverage Δ
Backend Apps	92.03% <ø> (+0.01%)	⬆️
Backend General	93.49% <ø> (ø)
Frontend	70.83% <ø> (ø)

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[matmair]

DO NOT MERGE; removes security feature

[codspeed-hq]

CodSpeed Performance Report

Merging this PR will not alter performance

_{Comparing matmair:feat--matrix-testing (e68e3b6) with master (2b09183)¹}

Summary

✅ 44 untouched benchmarks
⏩ 29 skipped benchmarks²

Footnotes

1. No successful run was found on master (c15805d) during the generation of this report, so 2b09183 was used instead as the comparison base. There might be some changes unrelated to this pull request in this report. ↩

2. 29 benchmarks were skipped, so the baseline results were used instead. If they were deleted from the codebase, click here and archive them to remove them from the performance reports. ↩

[matmair]

@SchrodingersGat ready for review and merge