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/lib.rs

@@ -26,3 +26,4 @@ pub mod agent_template;
 pub mod scheduled_task;
 pub mod telemetry;
 pub mod billing;
+pub mod knowledge;