feat(saas): wire llm_routing into account CRUD and auth responses

- Add llm_routing to all list_accounts/get_account SQL queries and JSON responses
- Add llm_routing to UpdateAccountRequest with COALESCE update
- Add llm_routing to AccountPublic struct in auth types
- Wire llm_routing into register (default 'local'), login, and me handlers
- Add llm_routing field to AccountRow, AccountAuthRow, AccountLoginRow model structs

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

crates/zclaw-saas/src/models/account.rs

@@ -14,6 +14,7 @@ pub struct AccountRow {
     pub totp_enabled: bool,
     pub last_login_at: Option<String>,
     pub created_at: String,
+    pub llm_routing: String,
 }
 
 /// accounts 表行 (不含 last_login_at，用于 auth/me 等场景)
@@ -27,6 +28,7 @@ pub struct AccountAuthRow {
     pub status: String,
     pub totp_enabled: bool,
     pub created_at: String,
+    pub llm_routing: String,
 }
 
 /// Login 一次性查询行（合并用户信息 + password_hash + totp_secret）
@@ -42,6 +44,7 @@ pub struct AccountLoginRow {
     pub password_hash: String,
     pub totp_secret: Option<String>,
     pub created_at: String,
+    pub llm_routing: String,
 }
 
 /// operation_logs 表行