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()),
         };

crates/zclaw-saas/src/main.rs

@@ -160,8 +160,9 @@ async fn main() -> anyhow::Result<()> {
                 interval.tick().await;
                 let pool = &metrics_db;
                 let total = pool.options().get_max_connections() as usize;
+                let size = pool.size() as usize;
                 let idle = pool.num_idle() as usize;
-                let used = total.saturating_sub(idle);
+                let used = size.saturating_sub(idle);
                 let usage_pct = if total > 0 { used * 100 / total } else { 0 };
                 tracing::info!(
                     "[PoolMetrics] total={} idle={} used={} usage_pct={}%",
@@ -248,9 +249,10 @@ async fn health_handler(
     let pool = &state.db;
     let total = pool.options().get_max_connections() as usize;
     if total > 0 {
+        let size = pool.size() as usize;
         let idle = pool.num_idle() as usize;
-        let used = total - idle;
-        let ratio = used * 100 / total;
+        let used = size.saturating_sub(idle);
+        let ratio = if size > 0 { used * 100 / total } else { 0 };
         if ratio >= 80 {
             return (
                 axum::http::StatusCode::SERVICE_UNAVAILABLE,