feat(saas): add knowledge base module — categories, items, versions, search, analytics

- 5 knowledge tables (categories, items, chunks, versions, usage) with pgvector + HNSW + GIN indexes
- 23+ API routes covering full CRUD, tree-structured categories, version snapshots
- Keyword-based search with ILIKE + array match (placeholder for vector search)
- Analytics endpoints: overview, trends, top-items, quality, gaps
- Markdown-aware content chunking with overlap strategy
- Worker dispatch for async embedding generation

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

crates/zclaw-saas/src/main.rs

@@ -337,6 +337,7 @@ async fn build_router(state: AppState) -> axum::Router {
         .merge(zclaw_saas::scheduled_task::routes())
         .merge(zclaw_saas::telemetry::routes())
         .merge(zclaw_saas::billing::routes())
+        .merge(zclaw_saas::knowledge::routes())
         .layer(middleware::from_fn_with_state(
             state.clone(),
             zclaw_saas::middleware::api_version_middleware,