feat(flywheel-c2-c3): C2 hasType4接真實API + C3 WebSocket指數退避重連

C2: flywheel_stats_service 加 type4_count query → API 回傳
    flywheel-diagram.tsx hasType4 改由 type4Count prop 驅動（非 false）
    flywheel-kpi-card.tsx 傳入 type4Count={flowData?.type4_count}

C3: WebSocket onclose 加指數退避重連 (1s→2s→4s→最大30s)
    cancelled 旗標確保 unmount 後不重連
    wsRetryTimer 加入 cleanup

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

apps/api/src/services/flywheel_stats_service.py

@@ -79,6 +79,7 @@ class FlywheelMetrics:
         self.node_stats = node_stats
         self.current_flow = current_flow
         self.computed_at = computed_at
+        self.type4_count: int = 0  # TYPE-4 incidents 數（ADR-073-C C2）
 
     def to_prometheus_lines(self) -> str:
         """輸出 Prometheus text format"""
@@ -115,6 +116,7 @@ class FlywheelMetrics:
         return {
             "nodes": self.node_stats,
             "current_flow": self.current_flow,
+            "type4_count": self.type4_count,
             "computed_at": self.computed_at.isoformat(),
         }
 
@@ -162,9 +164,10 @@ class FlywheelStatsService:
             today_processed,
             node_stats,
             current_flow,
+            type4_count,
         ) = await self._incident_stats(now)
 
-        return FlywheelMetrics(
+        metrics = FlywheelMetrics(
             playbook_count=playbook_count,
             execution_success_rate=execution_success_rate,
             km_unvectorized_count=km_unvectorized_count,
@@ -177,6 +180,8 @@ class FlywheelStatsService:
             current_flow=current_flow,
             computed_at=now,
         )
+        metrics.type4_count = type4_count
+        return metrics
 
     # ------------------------------------------------------------------
     # Internal helpers
@@ -274,6 +279,16 @@ class FlywheelStatsService:
                 )
                 incidents_stuck = stuck_q.scalar_one_or_none() or 0
 
+                # TYPE-4 Incident 數（ADR-073-C C2 — 供前端 hasType4 判斷）
+                # 2026-04-12 ogt
+                type4_q = await db.execute(
+                    select(func.count()).where(
+                        IncidentRecord.notification_type == "TYPE-4",
+                        IncidentRecord.status == IncidentStatus.INVESTIGATING.value,
+                    )
+                )
+                type4_count = type4_q.scalar_one_or_none() or 0
+
                 # 今日處理數
                 today_q = await db.execute(
                     select(func.count()).where(
@@ -355,11 +370,11 @@ class FlywheelStatsService:
                 },
             }
 
-            return alertname_null_rate, incidents_stuck, today_processed, node_stats, current_flow
+            return alertname_null_rate, incidents_stuck, today_processed, node_stats, current_flow, type4_count
 
         except Exception:
             logger.exception("flywheel_stats_incident_error")
-            return 0.0, 0, 0, {n: {"status": "unknown"} for n in FLYWHEEL_NODES}, []
+            return 0.0, 0, 0, {n: {"status": "unknown"} for n in FLYWHEEL_NODES}, [], 0
 
 
 def _status_to_node(status: str) -> str: