diff --git a/apps/api/src/db/base.py b/apps/api/src/db/base.py
index feebc32c..e629b588 100644
--- a/apps/api/src/db/base.py
+++ b/apps/api/src/db/base.py
@@ -242,6 +242,30 @@ async def init_db() -> None:
             """)
         )
 
+        # 2026-04-29 ogt + Claude Opus 4.7: M4 KMWriter 反查鏈 + 冪等補欄
+        # CD #1115-1117 全 failure 根因：commit c22e5f33 加 ORM 欄位但無對應 ALTER
+        #   錯誤：column "related_approval_id" of relation "knowledge_entries" does not exist
+        # 補防禦性 ALTER（同 timeline_events 模式）+ 對應 index
+        await conn.execute(
+            text("""
+            ALTER TABLE knowledge_entries
+                ADD COLUMN IF NOT EXISTS related_approval_id VARCHAR(64),
+                ADD COLUMN IF NOT EXISTS path_type VARCHAR(50);
+            """)
+        )
+        # M3 冪等 unique index (incident_id + path_type)
+        await conn.execute(text(
+            "CREATE UNIQUE INDEX IF NOT EXISTS uix_knowledge_incident_path "
+            "ON knowledge_entries(related_incident_id, path_type) "
+            "WHERE related_incident_id IS NOT NULL AND path_type IS NOT NULL;"
+        ))
+        # M4 反查鏈 partial index（approval_id → KM 反查）
+        await conn.execute(text(
+            "CREATE INDEX IF NOT EXISTS ix_knowledge_related_approval "
+            "ON knowledge_entries(related_approval_id) "
+            "WHERE related_approval_id IS NOT NULL;"
+        ))
+
         # W2 PR-L1 2026-04-28 ogt + Claude Sonnet 4.6: KM→Playbook 互饋回路（飛輪 C3 修復）
         # PlaybookRecord 新增 review_required 欄位
         # 已存在表不會被 create_all 重建，必須手動 ALTER