Add Knowledge Base V2

Structural health of a v3 knowledge base, computed at request time.

Returned on every GET /kbs/{id} so the FE can surface an inline banner + CTA when the KB needs repair. Four concrete issues can be detected by KBManagementService.compute_health:

• vector_table_missing — pgvector table was never created or was dropped out from under us. Repair rebuilds the table at the declared dim and queues every document for re-ingestion.
• index_struct_missing — the pgvector table is present but the kirby index_struct row is absent. Repair rebuilds the struct anchored to the existing column; no documents need reprocessing.
• dim_drift — declared dim no longer matches the pgvector column width. Retrieval still works against the actual dim, but new INSERTs fail. Repair recreates the table at the declared dim and queues every document for re-ingestion.
• resolver_error — the resolver itself failed (DB outage, unexpected column type). Not a repair-able state; the user is asked to retry later. HTTP stays 200 so the FE can render the apology inline instead of a blank error page.