fix(notifications): 新增後端 /api/v1/notifications/channels 路由

前端 /notifications 頁面呼叫此 endpoint 但後端不存在 (404)
新增 notifications.py：回傳 4 個真實頻道狀態
- Telegram OpenClaw Bot (BOT_TOKEN 設定檢查)
- Telegram Nemotron Bot (BOT_TOKEN 設定檢查)
- SSE Web Stream (永遠 active)
- Redis Stream awoooi:signals (ping 檢查)

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

apps/api/src/main.py

@@ -62,6 +62,7 @@ from src.api.v1 import (
 from src.api.v1 import drift as drift_v1  # Phase 25 P2: Config Drift Detection
 from src.api.v1 import rag as rag_v1  # Phase 33 ADR-067: RAG 知識庫
 from src.api.v1 import monitoring as monitoring_v1  # 2026-04-03: 監控工具狀態
+from src.api.v1 import notifications as notifications_v1  # 2026-04-10: 通知頻道狀態
 from src.api.v1 import stats as stats_v1  # Phase 6.5: Statistics Analytics
 from src.api.v1 import telegram as telegram_v1  # Phase 5.4: Telegram Gateway
 from src.api.v1 import terminal as terminal_v1  # Phase 19.1: Omni-Terminal SSE
@@ -506,6 +507,9 @@ app.include_router(
 app.include_router(
     signoz_webhook_v1.router, prefix="/api/v1", tags=["SignOz Webhook"]
 )  # Phase 21: SignOz → Telegram (ADR-037)
+app.include_router(
+    notifications_v1.router, prefix="/api/v1", tags=["Notifications"]
+)  # 2026-04-10: 通知頻道狀態
 app.include_router(
     terminal_v1.router, prefix="/api/v1", tags=["Omni-Terminal"]
 )  # Phase 19.1: Omni-Terminal SSE