Per-topic Kafka metadata for the topics list

Returns per-topic Kafka broker metadata (partition_count, replication_factor, retention_ms) and lastMessageTimestamp, keyed by topic_id.

sources/transforms entities must carry matching topic_ids and topic_db_ids (used for ownership verification); such entities missing topic_db_ids return HTTP 400. destinations are exempt (send topic_db_ids: []) — they are verified against the destination's topic_map. Topic names must match [a-zA-Z0-9._-]{1,249}; up to 1000 per entity.

All kafka.* fields are nullable — null means the broker did not return metadata for that topic, which is distinct from a legitimate 0. Values may be up to ~60 s stale for partition_count / replication_factor / retention_ms and up to ~10 s stale for lastMessageTimestamp.