[michaelwilner]

Which issue does this PR address?

Closes #4906

Rationale

The gateway fixes group.initial.rebalance.delay.ms at 3 seconds with no way to change it, while Kafka and Redpanda both let operators set it, down to 0.

What changed?

IGGY_KAFKA_GROUP_INITIAL_REBALANCE_DELAY_MS now sets GroupCoordinatorConfig::initial_rebalance_delay in milliseconds, validating the value is >= 0.

Added documentation to the README environment variable table and CONSUMER_GROUPS.md as well.

Local Execution

• Passed: cargo fmt -p iggy-gateway-kafka -- --check, cargo clippy -p iggy-gateway-kafka --all-targets --all-features -- -D warnings, cargo test -p iggy-gateway-kafka --bin iggy-gateway-kafka (24 passed, including the two new config tests), markdownlint on the two changed docs
• Pre-commit hooks not ran
• Ran the binaries as part of our own integration test suite that stands up a Kafka broker already to exercise the full system end to end

AI Usage

Used Claude to research conventions to match with the rest of the repo and generate tests.

[github-actions]

Thanks for the PR. It is labeled S-waiting-on-review and queued for review.

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[hubcio]

@krishvishal can u check this one?