refactor(api): Phase 16 R2 封存死代碼 + RiskLevel 統一

封存 (866 行):
- routes/approvals.py → _archived/routes/ (477 行，未註冊死代碼)
- services/approval.py → _archived/services/ (389 行，僅被死代碼使用)

合併 RiskLevel:
- models/approval.py 新增 HIGH (從 trust_engine.py 合併)
- trust_engine.py 改 import from models/approval.py
- 保留舊定義為註解供回滾

更新 services/__init__.py:
- 移除已封存模組的 import (註解保留回滾路徑)

驗證:
- RiskLevel 統一: models 與 trust_engine 使用同一 class
- 24 個 action_parsing 測試通過

回滾指令見 _archived/README.md

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

apps/api/src/models/approval.py

@@ -43,10 +43,15 @@ class RiskLevel(str, Enum):
 
     - LOW: 0 人，自動放行
     - MEDIUM: 需 1 人簽核
-    - CRITICAL: 需 2 人 Multi-Sig 雙重簽核
+    - HIGH: 需 1 人簽核 (信任引擎可降級至 MEDIUM)
+    - CRITICAL: 需 2 人 Multi-Sig 雙重簽核 (永不降級)
+
+    變更紀錄:
+    - 2026-03-25: 新增 HIGH (Phase 16 R2 合併自 trust_engine.py)
     """
     LOW = "low"
     MEDIUM = "medium"
+    HIGH = "high"  # Phase 16 R2: 從 trust_engine.py 合併 (2026-03-25)
     CRITICAL = "critical"