fix(adr075): 修補斷點E — _push_to_telegram_background 補 TYPE-8M routing

斷點E：alertmanager webhook 走 _push_to_telegram_background，
未含 TYPE-8M branch，導致 meta alert 從未送出。

- webhooks.py: 新增 alert_category 參數 + TYPE-8M branch
- incident_service.py: 還原 rule 5 僅攔 watchdog/heartbeat，
  移除誤加的 backup startswith 規則（VeleroBackup 由 K8s rule 接管）

Tests: 52/52 passed

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

apps/api/src/services/incident_service.py

@@ -164,14 +164,8 @@ def classify_alert_early(alertname: str, severity: str, labels: dict | None = No
         return "info", "TYPE-1"
 
     # 5. Backup / Heartbeat — 純資訊，不進 LLM
-    # HostBackupFailed 必須在 Host prefix 前攔截，否則被歸 host_resource/TYPE-3
-    # 2026-04-12 ogt: 補充 HostBackup/Backup/VeleroBackup 前綴
-    if (
-        "watchdog" in alertname_lower
-        or alertname == "Heartbeat"
-        or alertname.startswith(("HostBackup", "Backup", "VeleroBackup", "BackupRestore"))
-        or "backup" in alertname_lower
-    ):
+    # VeleroBackup 由 K8s prefix 規則接管，此處只攔 watchdog/heartbeat
+    if "watchdog" in alertname_lower or alertname == "Heartbeat":
         return "backup", "TYPE-1"
 
     # 6. 主機資源（從 infrastructure 分離，ADR-075 統帥決議）