feat(cogs): report backend changes to a change stream

[matt-codecov]

rig up GCS and Bigtable backends to emit change stream records. nothing is actually plugging in a change stream yet, that'll be future PRs

didn't do other backends yet because i wanted feedback on the stack so far before writing that much more code haha

[codecov]

Codecov Report

❌ Patch coverage is 96.55172% with 10 lines in your changes missing coverage. Please review.
✅ Project coverage is 88.32%. Comparing base (f819003) to head (959eb00).

Files with missing lines	Patch %	Lines
objectstore-service/src/backend/change_stream.rs	0.00%	6 Missing ⚠️
objectstore-service/src/backend/bigtable.rs	98.75%	2 Missing ⚠️
objectstore-service/src/backend/gcs.rs	98.19%	2 Missing ⚠️

Additional details and impacted files

@@                          Coverage Diff                          @@
##           matth/storage-inventory-tracker-2     #595      +/-   ##
=====================================================================
+ Coverage                              88.12%   88.32%   +0.19%     
=====================================================================
  Files                                    100      101       +1     
  Lines                                  16448    16875     +427     
=====================================================================
+ Hits                                   14495    14905     +410     
- Misses                                  1953     1970      +17     

Components	Coverage Δ
Rust Backend	92.44% <96.55%> (+0.10%)	⬆️
Rust Client	81.97% <ø> (ø)
Python Client	93.31% <ø> (ø)

☔ View full report in Codecov by Harness.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.

[cursor]

Cursor Bugbot has reviewed your changes and found 1 potential issue.

^{❌ Bugbot Autofix is OFF. To automatically fix reported issues with cloud agents, enable autofix in the Cursor dashboard.}

^{Reviewed by Cursor Bugbot for commit 959eb00. Configure here.}

[cursor]

Multipart uploads bypass change stream reporting

Medium Severity

GcsBackend reports writes only from put_object, so objects finalized through complete_multipart never reach the ChangeStream. Large uploads — exactly the ones routed to the long-term tier — stay invisible in the inventory, while the matching Bigtable tombstone is reported, and a later delete_object emits a delete for a record that was never written. Storage COGS then undercounts long-term usage.

Additional Locations (1)

• objectstore-service/docs/architecture.md#L139-L152

 

^{Reviewed by Cursor Bugbot for commit 959eb00. Configure here.}