diff --git a/apps/api/src/_archived/README.md b/apps/api/src/_archived/README.md
new file mode 100644
index 00000000..0ee2dad0
--- /dev/null
+++ b/apps/api/src/_archived/README.md
@@ -0,0 +1,41 @@
+# Archived Code (Phase 16 R2)
+
+> **封存日期**: 2026-03-25
+> **封存者**: Claude Code (Phase 16 R2)
+> **策略**: 絞殺者模式 (Strangler Fig Pattern)
+
+---
+
+## 封存原因
+
+這些檔案已被新版本取代，但保留供參考與回滾。
+
+### routes/approvals.py (477 行)
+- **取代者**: `api/v1/approvals.py`
+- **原因**: 未註冊到 main.py，屬於死代碼
+- **回滾**: 如需恢復，移回 `src/routes/` 並在 main.py 註冊
+
+### services/approval.py (389 行)
+- **取代者**: `services/approval_db.py` + `services/approval_execution.py`
+- **原因**: 只被 routes/approvals.py 使用，隨之封存
+- **回滾**: 如需恢復，移回 `src/services/`
+
+---
+
+## 回滾指令
+
+```bash
+# 回滾 routes/approvals.py
+git mv apps/api/src/_archived/routes/approvals.py apps/api/src/routes/approvals.py
+
+# 回滾 services/approval.py
+git mv apps/api/src/_archived/services/approval.py apps/api/src/services/approval.py
+```
+
+---
+
+## 清理時間表
+
+- **封存**: 2026-03-25
+- **觀察期**: 30 天 (至 2026-04-25)
+- **正式刪除**: 觀察期後無問題可永久刪除
diff --git a/apps/api/src/routes/approvals.py b/apps/api/src/_archived/routes/approvals.py
similarity index 100%
rename from apps/api/src/routes/approvals.py
rename to apps/api/src/_archived/routes/approvals.py
diff --git a/apps/api/src/services/approval.py b/apps/api/src/_archived/services/approval.py
similarity index 100%
rename from apps/api/src/services/approval.py
rename to apps/api/src/_archived/services/approval.py
diff --git a/apps/api/src/models/approval.py b/apps/api/src/models/approval.py
index 853b4419..7e2137be 100644
--- a/apps/api/src/models/approval.py
+++ b/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"
 
 
diff --git a/apps/api/src/services/__init__.py b/apps/api/src/services/__init__.py
index 1e92edfb..74ea559f 100644
--- a/apps/api/src/services/__init__.py
+++ b/apps/api/src/services/__init__.py
@@ -1,23 +1,28 @@
 """
 AWOOOI API Services
+
+變更紀錄:
+- 2026-03-25 Phase 16 R2: 移除 .approval import (已封存至 _archived/)
 """
 
-from .approval import (
-    RISK_MATRIX,
-    ApprovalAlreadyDecidedError,
-    # Exceptions
-    ApprovalError,
-    ApprovalNotFoundError,
-    ApprovalState,
-    ApprovalStatus,
-    DuplicateSignatureError,
-    InsufficientPermissionError,
-    MultiSigEngine,
-    Signature,
-    TOCTOUConflictError,
-    UserRole,
-    multi_sig_engine,
-)
+# Phase 16 R2 (2026-03-25): 以下 import 已封存，原模組移至 _archived/services/approval.py
+# 回滾: 將 _archived/services/approval.py 移回，取消下方註解
+# from .approval import (
+#     RISK_MATRIX,
+#     ApprovalAlreadyDecidedError,
+#     ApprovalError,
+#     ApprovalNotFoundError,
+#     ApprovalState,
+#     ApprovalStatus,
+#     DuplicateSignatureError,
+#     InsufficientPermissionError,
+#     MultiSigEngine,
+#     Signature,
+#     TOCTOUConflictError,
+#     UserRole,
+#     multi_sig_engine,
+# )
+
 from .consensus_engine import (
     AgentOpinion,
     AgentType,
@@ -54,21 +59,20 @@ __all__ = [
     "DryRunEngine",
     "DryRunResult",
     "dry_run_engine",
-    # Multi-Sig
-    "MultiSigEngine",
-    "multi_sig_engine",
-    "ApprovalState",
-    "Signature",
-    "UserRole",
-    "ApprovalStatus",
-    "RISK_MATRIX",
-    # Exceptions
-    "ApprovalError",
-    "InsufficientPermissionError",
-    "DuplicateSignatureError",
-    "TOCTOUConflictError",
-    "ApprovalNotFoundError",
-    "ApprovalAlreadyDecidedError",
+    # Phase 16 R2: Multi-Sig 已封存至 _archived/services/approval.py
+    # "MultiSigEngine",
+    # "multi_sig_engine",
+    # "ApprovalState",
+    # "Signature",
+    # "UserRole",
+    # "ApprovalStatus",
+    # "RISK_MATRIX",
+    # "ApprovalError",
+    # "InsufficientPermissionError",
+    # "DuplicateSignatureError",
+    # "TOCTOUConflictError",
+    # "ApprovalNotFoundError",
+    # "ApprovalAlreadyDecidedError",
     # Trust Engine
     "TrustScoreManager",
     "trust_engine",
diff --git a/apps/api/src/services/trust_engine.py b/apps/api/src/services/trust_engine.py
index 4e5004bb..f93c6017 100644
--- a/apps/api/src/services/trust_engine.py
+++ b/apps/api/src/services/trust_engine.py
@@ -19,7 +19,7 @@ Phase 3.2: Progressive Autonomy
 import logging
 from dataclasses import dataclass, field
 from datetime import datetime
-from enum import Enum
+# from enum import Enum  # Phase 16 R2: 不再需要，RiskLevel 改從 models 導入
 from typing import Literal
 
 logger = logging.getLogger(__name__)
@@ -27,13 +27,20 @@ logger = logging.getLogger(__name__)
 
 # ==================== Types ====================
 
+# Phase 16 R2 (2026-03-25): RiskLevel 統一改從 models/approval.py 導入
+# 原因: 消除重複定義，統一風險等級來源
+# 執行者: Claude Code
+# 回滾: 取消註解下方區塊，移除 import
+from src.models.approval import RiskLevel
 
-class RiskLevel(str, Enum):
-    """風險等級"""
-    LOW = "low"
-    MEDIUM = "medium"
-    HIGH = "high"
-    CRITICAL = "critical"
+# --- 以下為舊定義，已封存 (Phase 16 R2) ---
+# class RiskLevel(str, Enum):
+#     """風險等級"""
+#     LOW = "low"
+#     MEDIUM = "medium"
+#     HIGH = "high"
+#     CRITICAL = "critical"
+# --- 封存結束 ---
 
 
 @dataclass