feat(cogs): publish backend change streams to Kafka

[matt-codecov]

connect SinkConfigs to objectstore-server conf and wire it up to the ChangeStreamFactory. this completes the objectstore code changes for storage COGS

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[codecov]

Codecov Report

❌ Patch coverage is 94.87179% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 88.46%. Comparing base (28100c2) to head (d5d5bef).

Files with missing lines	Patch %	Lines
objectstore-server/src/config.rs	94.59%	2 Missing ⚠️

Additional details and impacted files

@@                        Coverage Diff                         @@
##           matth/storage-inventory-tracker-3     #597   +/-   ##
==================================================================
  Coverage                              88.46%   88.46%           
==================================================================
  Files                                    104      104           
  Lines                                  17055    17092   +37     
==================================================================
+ Hits                                   15087    15121   +34     
- Misses                                  1968     1971    +3     

Components	Coverage Δ
Rust Backend	92.47% <94.87%> (+<0.01%)	⬆️
Rust Client	81.97% <ø> (ø)
Python Client	93.31% <ø> (ø)

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[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