fix: optimize transducer observation query by using subquery for latest blocks

[jirhiker]

Why

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How

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Notes

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[codecov-commenter]

Codecov Report

❌ Patch coverage is 60.00000% with 4 lines in your changes missing coverage. Please review.
✅ All tests successful. No failed tests found.

Files with missing lines	Patch %	Lines
services/observation_helper.py	60.00%	4 Missing ⚠️

Files with missing lines	Coverage Δ
services/observation_helper.py	94.11% <60.00%> (+2.07%)	⬆️

... and 4 files with indirect coverage changes