node: Wire up ChainSection.ingestor to control block ingestion

The `[chains] ingestor` config field was validated and logged but never
actually consulted when deciding whether to run block ingestion. Add
`Config::is_block_ingestor()` as the single source of truth: ingestion
runs only when `--disable-block-ingestor` is not set and the node's id
matches the configured ingestor value.