feat(cogs): publish backend change streams to Kafka

[matt-codecov]

Ref FS-210

Add CostTrackerConfig struct to objectstore-server's config. This allows storage COGS to be configured end-to-end; CostTrackerConfig on objectstore-server's config and CostTrackerStreamConfig on each Backend impl will produce a CostTrackerStream that holds a shared Kafka producer and streams change messages on the shared-resources-inventory topic.

While ChangeStream is abstract, the abstraction is not exposed in config. It's assumed each ChangeStream impl will name itself explicitly in dedicated config sections.

---

_{Stack created with GitHub Stacks CLI • Give Feedback 💬}

[codecov]

Codecov Report

❌ Patch coverage is 95.65217% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 88.46%. Comparing base (da74fdb) to head (9951ac3).

Files with missing lines	Patch %	Lines
objectstore-server/src/config.rs	95.00%	2 Missing ⚠️

Additional details and impacted files

@@                          Coverage Diff                          @@
##           matth/storage-inventory-tracker-3     #597      +/-   ##
=====================================================================
+ Coverage                              88.39%   88.46%   +0.07%     
=====================================================================
  Files                                    104      104              
  Lines                                  17118    17162      +44     
=====================================================================
+ Hits                                   15131    15182      +51     
+ Misses                                  1987     1980       -7     

Components	Coverage Δ
Rust Backend	92.48% <95.65%> (+0.03%)	⬆️
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.

[sentry]

Bug: A partial Kafka configuration with a topic but no bootstrap_servers will be accepted at startup but will silently fail to send messages at runtime.
_{Severity: HIGH}

Suggested Fix

Validate the SinkConfig to ensure that if the Kafka sink is enabled, the bootstrap_servers field is not empty. This check should be performed during configuration loading to cause a startup failure with a clear error message, preventing silent failures in production.

Prompt for AI Agent

Review the code at the location below. A potential bug has been identified by an AI
agent. Verify if this is a real issue. If it is, propose a fix; if not, explain why it's
not valid.

Location: objectstore-server/src/config.rs#L930

Potential issue: If the service is configured with a partial Kafka configuration,
specifically providing a topic but omitting the `bootstrap_servers`, the application
will start without error. The underlying `librdkafka` library accepts an empty list of
bootstrap servers during producer creation, so no startup error is logged. However, the
producer will be unable to connect to any brokers, causing all change stream messages to
be silently dropped. This can lead to an operator being unaware that their intended
configuration is not active, resulting in a loss of inventory tracking data.

Also affects:

• objectstore-inventory-tracker/src/kafka.rs:105
• objectstore-service/src/change_stream/kafka.rs:80

[linear-code]

FS-210