ref(service): Inject access time into storage operations

[jan-auer]

Capture one timestamp per HTTP request and pass it through metadata creation, service calls, and backend expiry checks. Previously, backends sampled time independently. A slow request could therefore use different times for creation, expiry checks, and TTI renewal.

Use an explicit timestamp parameter to keep the interface small and dependencies visible. Endpoints extract the timestamp, and the auth wrapper forwards it like the other operation arguments. Reuse it across all batch items, tiered calls, and retries.

Background renewals and cleanup start separate operations and use fresh timestamps for liveness checks. This prevents time spent waiting in a queue from authorizing renewal of an expired object. Renewal deadlines still come from the original read, so queue delays do not extend retention.

Keep multipart modification timestamps and cleanup grace periods unchanged. They serve separate purposes from object access and expiry.

[codecov]

Codecov Report

❌ Patch coverage is 97.41176% with 33 lines in your changes missing coverage. Please review.
✅ Project coverage is 89.49%. Comparing base (50f276d) to head (486d015).

Files with missing lines	Patch %	Lines
objectstore-service/src/backend/s3_compatible.rs	7.14%	26 Missing ⚠️
objectstore-server/src/endpoints/resumable.rs	63.63%	4 Missing ⚠️
bigtable-bench/src/main.rs	0.00%	3 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #630      +/-   ##
==========================================
+ Coverage   89.15%   89.49%   +0.34%     
==========================================
  Files         113      114       +1     
  Lines       20692    21564     +872     
==========================================
+ Hits        18447    19298     +851     
- Misses       2245     2266      +21     

Components	Coverage Δ
Rust Backend	93.75% <97.64%> (+0.21%)	⬆️
Rust Client	72.70% <ø> (ø)
Python Client	93.56% <ø> (ø)

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[lcian]

As we discussed, I don't foresee any cases where we would need to carry around more "request context" other than this timestamp, and I think carrying this in a task-local or similar implicit "context" would likely be error-prone. So, I'm fine with this approach.