feat(api): ActionType.SSH_COMMAND + auto_repair_service SSH分支 (Task 12)

- playbook.py: 新增 SSH_COMMAND ActionType
- auto_repair_service._execute_step: SSH_COMMAND 分支，格式 layer/component

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

apps/api/src/models/playbook.py

@@ -47,6 +47,7 @@ class ActionType(str, Enum):
     KUBECTL = "kubectl"  # Kubernetes 命令
     SCRIPT = "script"  # 腳本執行
     MANUAL = "manual"  # 需人工操作
+    SSH_COMMAND = "ssh_command"  # 2026-04-05 Claude Code: 主機層 SSH 修復 (Sprint 3)
 
 
 class RiskLevel(str, Enum):