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/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,