feat(c3): ADR-073-C C3 — WebSocket 飛輪即時推送

後端：
- stats.py 新增 @router.websocket('/flywheel/ws')
- 每 10 秒推送 flywheel_summary JSON

前端 FlywheelKPICard：
- WebSocket 優先，WS 斷線自動降級到 30s HTTP 輪詢
- onopen 時停止 HTTP polling，onclose 時恢復

2026-04-12 ogt (ADR-073-C C3)

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

apps/api/src/api/v1/stats.py

@@ -19,9 +19,11 @@
 # @see feedback_lewooogo_modular_enforcement.md
 # =============================================================================
 
+import asyncio
+import json
 from typing import Annotated, Any
 
-from fastapi import APIRouter, Depends, Query
+from fastapi import APIRouter, Depends, Query, WebSocket, WebSocketDisconnect
 from fastapi.responses import PlainTextResponse
 from pydantic import BaseModel, Field
 
@@ -552,3 +554,35 @@ async def get_flywheel_prometheus_metrics(svc: FlywheelStatsDep) -> PlainTextRes
         content=metrics.to_prometheus_lines(),
         media_type="text/plain; version=0.0.4; charset=utf-8",
     )
+
+
+# =============================================================================
+# ADR-073-C C3: WebSocket 即時飛輪推送
+# =============================================================================
+
+@router.websocket("/flywheel/ws")
+async def flywheel_websocket(websocket: WebSocket) -> None:
+    """
+    WebSocket 即時飛輪健康度推送 — ADR-073-C C3
+
+    每 10 秒推送一次 FlywheelSummary JSON。
+    前端連線路徑：ws(s)://<host>/api/v1/stats/flywheel/ws
+
+    Protocol:
+      Server → Client: {"type": "flywheel_summary", "data": {...}, "ts": "ISO8601"}
+      Client → Server: (ignored)
+    """
+    svc = get_flywheel_stats_service()
+    await websocket.accept()
+    try:
+        while True:
+            metrics = await svc.compute()
+            payload = json.dumps({
+                "type": "flywheel_summary",
+                "data": metrics.to_summary_api_dict(),
+                "ts": metrics.computed_at.isoformat(),
+            })
+            await websocket.send_text(payload)
+            await asyncio.sleep(10)
+    except WebSocketDisconnect:
+        pass