fix(service): Scale TTI debounce proportionally for short TTI values

[jan-auer]

The fixed 24-hour TTI_DEBOUNCE constant silently prevented TTI bumps for any TTI value of 24 hours or less — the bump condition expire_at < now + tti - 24h could never be satisfied, so short-TTI objects degraded into fixed-TTL without any indication.

The debounce window now scales with the TTI duration:

• TTI below 4 days: uses tti / 4 — e.g. a 2-hour TTI gets a 30-minute debounce so bumps actually fire.
• TTI of 4 days or more: stays at 24 hours, preserving existing behavior.

Fixes FS-403

[linear-code]

FS-403

[codecov]

Codecov Report

❌ Patch coverage is 95.38462% with 6 lines in your changes missing coverage. Please review.
✅ Project coverage is 87.58%. Comparing base (56d980d) to head (f2a87c5).

Files with missing lines	Patch %	Lines
objectstore-service/src/backend/s3_compatible.rs	0.00%	6 Missing ⚠️

Additional details and impacted files

@@                       Coverage Diff                        @@
##           fix/s3-expired-object-filter     #571      +/-   ##
================================================================
+ Coverage                         87.46%   87.58%   +0.12%     
================================================================
  Files                                93       93              
  Lines                             15152    15260     +108     
================================================================
+ Hits                              13252    13365     +113     
+ Misses                             1900     1895       -5     

Components	Coverage Δ
Rust Backend	92.27% <95.38%> (+0.12%)	⬆️
Rust Client	79.89% <ø> (ø)
Python Client	90.20% <ø> (ø)

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