fix(saas): harden model group failover + relay reliability

- cache: insert-then-retain pattern avoids empty-window race during refresh
- relay: manage_task_status flag for proper failover state transitions
- relay: retry_task re-resolves model groups instead of blind provider reuse
- relay: filter empty-member groups from available models list
- relay: quota cache stale entry cleanup (TTL 5x expiry)
- error: from_sqlx_unique helper for 409 vs 500 distinction
- model_config: unique constraint handling, duplicate member check
- model_config: failover_strategy whitelist, model_id vs group name conflict check
- model_config: group-scoped member removal with group_id validation

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

crates/zclaw-saas/src/error.rs

@@ -64,6 +64,18 @@ pub enum SaasError {
 }
 
 impl SaasError {
+    /// 将 sqlx::Error 中的 unique violation 映射为 AlreadyExists (409)，
+    /// 其他 DB 错误保持为 Database (500)。
+    pub fn from_sqlx_unique(e: sqlx::Error, context: &str) -> Self {
+        if let sqlx::Error::Database(ref db_err) = e {
+            // PostgreSQL unique_violation = "23505"
+            if db_err.code().map(|c| c == "23505").unwrap_or(false) {
+                return Self::AlreadyExists(format!("{}已存在", context));
+            }
+        }
+        Self::Database(e)
+    }
+
     /// 获取 HTTP 状态码
     pub fn status_code(&self) -> StatusCode {
         match self {