ref: Use Duration helper methods for hours and minutes

[jan-auer]

Replace Duration::from_secs arithmetic with Duration::from_hours and Duration::from_mins where the intent is hours or minutes. Expressions like from_secs(2 * 24 * 3600) become from_hours(2 * 24), and from_secs(3600) becomes from_hours(1).

Duration::from_days would be preferable in some cases (e.g., from_hours(90 * 24)) but it's still nightly-only (duration_constructors_lite, #141881).

[codecov]

Codecov Report

❌ Patch coverage is 98.11321% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 87.38%. Comparing base (c74e688) to head (851a9ca).

Files with missing lines	Patch %	Lines
bigtable-bench/src/main.rs	0.00%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #503   +/-   ##
=======================================
  Coverage   87.38%   87.38%           
=======================================
  Files          86       86           
  Lines       13825    13825           
=======================================
  Hits        12081    12081           
  Misses       1744     1744           

Components	Coverage Δ
Rust Backend	92.19% <100.00%> (ø)
Rust Client	80.06% <ø> (ø)
Python Client	88.51% <ø> (ø)

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