feat: add AI catalog description generation

docs/architecture/overview.md

@@ -12,6 +12,8 @@ flowchart LR
     USER["Reviewer"] --> FE["Frontend\nReact and SSE"]
     FE --> BE["Backend\nFastify"]
     BE --> CATALOG["Metadata catalog\nPostgreSQL"]
+    BE --> MODEL["Configured AI model\nvia short-lived LiteLLM helper"]
+    BE -->|bounded read-only samples| DWH
     BE --> PI["Pi\nRPC per sessione"]
     PI --> THT["tht and harness\nworkflow and persistence"]
     THT --> DWH["DWH\nread only"]
@@ -50,12 +52,19 @@ A session is a directory under `sessions/` (the workspace defines the path): `se
   configurations stored in PostgreSQL through Kysely.
 - `CatalogTableService` reconciles persisted Catalog Tables with a successful external schema scan;
   `ConcreteCatalogTableIntrospector` isolates direct PostgreSQL, typed REST, and SSH-tunnel access.
+- `DescriptionGenerationWorker` admits one installation-wide run and processes its table or column
+  targets sequentially.
+- `PostgresDescriptionSourceSampler` reads bounded real rows and five representative examples from
+  the configured DWH connection; `ProcessModelCompleter` invokes the short-lived Python LiteLLM
+  helper with the installation-selected model.
 
 Application settings remain in `backend/data/settings.json`; session state remains in harness phase
 documents. PostgreSQL stores only the administrative database catalog, bindings, observed tables,
-and curated descriptions. Connector secrets remain write-only in the encrypted workspace secret
-store. Catalog SSH support is limited to connection tests and table synchronization; it does not
-change the session runtime binding contract.
+curated and generated descriptions, description-generation runs, and sanitized run events.
+Connector secrets remain write-only in the encrypted workspace secret store; model credentials
+remain in the protected installation secret bundle. Catalog SSH support is limited to connection
+tests, table synchronization, and bounded description-generation sampling; it does not change the
+session runtime binding contract.
 
 ## Human-in-the-loop gate contract