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/db.rs

@@ -5,7 +5,7 @@ use sqlx::PgPool;
 use crate::config::DatabaseConfig;
 use crate::error::SaasResult;
 
-const SCHEMA_VERSION: i32 = 12;
+const SCHEMA_VERSION: i32 = 13;
 
 /// 初始化数据库
 pub async fn init_db(config: &DatabaseConfig) -> SaasResult<PgPool> {