fix(saas): 新用户 llm_routing 默认改为 relay 使 SaaS token pool 成为主路径

- handlers.rs: SQL INSERT 和 LoginResponse 中 'local' → 'relay'
- 新增 migration: ALTER llm_routing SET DEFAULT 'relay'
- 符合管家式服务理念：用户无需配置 API Key，SaaS 自动中转

crates/zclaw-saas/migrations/20260411000001_accounts_llm_routing_default_relay.sql

@@ -0,0 +1,3 @@
+-- 20260411000001_accounts_llm_routing_default_relay.sql
+-- 新用户默认走 SaaS relay (Token Pool), 符合管家式服务理念
+ALTER TABLE accounts ALTER COLUMN llm_routing SET DEFAULT 'relay';

crates/zclaw-saas/src/auth/handlers.rs

@@ -120,7 +120,7 @@ pub async fn register(
 
     sqlx::query(
         "INSERT INTO accounts (id, username, email, password_hash, display_name, role, status, created_at, updated_at, llm_routing)
-         VALUES ($1, $2, $3, $4, $5, $6, 'active', $7, $7, 'local')"
+         VALUES ($1, $2, $3, $4, $5, $6, 'active', $7, $7, 'relay')"
     )
     .bind(&account_id)
     .bind(&req.username)
@@ -176,7 +176,7 @@ pub async fn register(
             status: "active".into(),
             totp_enabled: false,
             created_at: now.to_rfc3339(),
-            llm_routing: "local".into(),
+            llm_routing: "relay".into(),
         },
     };
     let jar = set_auth_cookies(jar, &resp.token, &refresh_token);