fix(storage): bound coverage cleanup batch

[fewensa]

Summary

• reduce coverage-index-v2 cleanup records per compaction tick from 32 to 4
• keep cleanup safety validation and read-side delta budget unchanged
• update maintenance tests for the smaller cleanup batch

Production evidence

• Arbitrum compaction tick started, then did not emit a completed/failed log after cleanup scan
• the last observed scan had eligible_records=32 max_records=32, so cleanup work could monopolize the tick before hot bucket compaction progressed

Correctness

• does not raise or disable the read-side 128 delta-object guard
• does not skip snapshot/head safety checks before deleting old deltas
• does not delete S3/RustFS objects manually

Tests

• cargo fmt --check
• cargo test -p datalens-storage --test maintenance test_compaction_coverage_index_v2 -- --nocapture
• cargo clippy -p datalens-storage --all-targets -- -D warnings