feat(sequences): add offset query param to detections endpoint

[ThbltLmr]

Summary

• Adds an offset query parameter to the GET /sequences/{sequence_id}/detections endpoint, wiring through the existing fetch_all offset support so callers can page through all detections of a sequence.

Test plan

• New tests in src/tests/endpoints/test_sequences.py cover the offset behavior
• Verify pagination works end-to-end against a populated sequence

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 90.15%. Comparing base (3a69b32) to head (b18ab1c).

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #592      +/-   ##
==========================================
+ Coverage   90.11%   90.15%   +0.04%     
==========================================
  Files          55       55              
  Lines        2498     2498              
==========================================
+ Hits         2251     2252       +1     
+ Misses        247      246       -1     

Flag	Coverage Δ
backend	90.13% <ø> (+0.04%)	⬆️
client	90.40% <ø> (ø)

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