fix: saasStore require() bug + health check pool formula + DEV error details

- saasStore.ts: replace require('./chat/conversationStore') with await import()
  to fix ReferenceError in Vite ESM environment (P1)
- main.rs: fix health check pool usage formula from max_connections - num_idle
  to pool.size() - num_idle, preventing false "degraded" status (P1)
- error.rs: show detailed error messages in ZCLAW_SAAS_DEV=true mode
- Update bug tracker with BUG-003 through BUG-007

crates/zclaw-saas/src/error.rs

@@ -127,11 +127,15 @@ impl IntoResponse for SaasError {
     fn into_response(self) -> Response {
         let status = self.status_code();
         let (error_code, message) = match &self {
-            // 500 错误不泄露内部细节给客户端
+            // 500 错误不泄露内部细节给客户端 (开发模式除外)
             Self::Database(_) | Self::Internal(_) | Self::Io(_)
             | Self::Jwt(_) | Self::Config(_) => {
                 tracing::error!("内部错误 [{}]: {}", self.error_code(), self);
-                (self.error_code().to_string(), "服务内部错误".to_string())
+                if std::env::var("ZCLAW_SAAS_DEV").as_deref() == Ok("true") {
+                    (self.error_code().to_string(), format!("[DEV] {}", self))
+                } else {
+                    (self.error_code().to_string(), "服务内部错误".to_string())
+                }
             }
             _ => (self.error_code().to_string(), self.to_string()),
         };