[KM-564] Edit column description in catalog to reduce token & ingestion time

[NOTICKET] cleanup PR: wire ChatHandler, drop Phase 1 remnants, implement catalog rebuild

[KM-560][KM-561] drop catalog LLM enrichment + rename store to data_catalog + add /data-catalog index endpoint

TabularIntrospector + on_tabular_uploaded trigger + 31 tests

[NOTICKET] catalog enricher + structured pipeline + on_db_registered

[KM-553] initialize shared contract

initial structure

initial commit