fix(ops): make bge-m3 migration idempotent

apps/api/migrations/embedding_bge_m3_1024.sql

@@ -10,7 +10,8 @@
 --   2. rag_chunks.embedding          vector(768) → vector(1024)
 --   3. playbook_embeddings.embedding vector(768) → vector(1024)
 --
--- 遷移策略：清空現有向量資料，切換維度後由 re-embed script 重新嵌入
+-- 遷移策略：僅在欄位不是 vector(1024) 時清空現有向量資料，切換維度後由 re-embed script 重新嵌入
+-- 已經是 vector(1024) 的環境重跑本 migration 時，必須保留既有向量資料。
 -- 現有向量資料若要保留，需先 dump 用 nomic 格式備份（舊維度無法轉換）
 --
 -- 執行前置條件：
@@ -23,17 +24,38 @@
 BEGIN;
 
 -- 1. knowledge_entries：備份舊向量並清空，變更欄位維度
-CREATE TABLE IF NOT EXISTS knowledge_entries_embedding_backup_20260505 AS
-SELECT
-    id,
-    embedding::text AS embedding_768,
-    NOW() AS backed_up_at
-FROM knowledge_entries
-WHERE embedding IS NOT NULL;
+DO $$
+DECLARE
+    v_dim integer;
+BEGIN
+    SELECT a.atttypmod INTO v_dim
+    FROM pg_attribute a
+    JOIN pg_class c ON a.attrelid = c.oid
+    WHERE c.relname = 'knowledge_entries'
+      AND a.attname = 'embedding';
 
-ALTER TABLE knowledge_entries
-    ALTER COLUMN embedding TYPE vector(1024)
-    USING NULL;  -- 清空現有 768 維向量（維度不可轉換）
+    IF v_dim IS DISTINCT FROM 1024 THEN
+        EXECUTE $sql$
+            CREATE TABLE IF NOT EXISTS knowledge_entries_embedding_backup_20260505 AS
+            SELECT
+                id,
+                embedding::text AS embedding_768,
+                NOW() AS backed_up_at
+            FROM knowledge_entries
+            WHERE embedding IS NOT NULL
+        $sql$;
+
+        EXECUTE $sql$
+            ALTER TABLE knowledge_entries
+                ALTER COLUMN embedding TYPE vector(1024)
+                USING NULL
+        $sql$;
+
+        RAISE NOTICE 'knowledge_entries.embedding migrated from vector(%) to vector(1024); old embeddings were backed up and cleared', v_dim;
+    ELSE
+        RAISE NOTICE 'knowledge_entries.embedding already vector(1024); existing embeddings preserved';
+    END IF;
+END $$;
 
 COMMENT ON COLUMN knowledge_entries.embedding IS
     'bge-m3:latest 1024 維向量 — 遷移自 nomic-embed-text 768 維 (2026-05-05 ADR-110 follow-up)';
@@ -41,11 +63,29 @@ COMMENT ON COLUMN knowledge_entries.embedding IS
 
 -- 2. rag_chunks：清空向量資料，變更欄位維度
 --    ivfflat index 必須先 DROP 才能 ALTER COLUMN
-DROP INDEX IF EXISTS idx_rag_chunks_embedding;
+DO $$
+DECLARE
+    v_dim integer;
+BEGIN
+    SELECT a.atttypmod INTO v_dim
+    FROM pg_attribute a
+    JOIN pg_class c ON a.attrelid = c.oid
+    WHERE c.relname = 'rag_chunks'
+      AND a.attname = 'embedding';
 
-ALTER TABLE rag_chunks
-    ALTER COLUMN embedding TYPE vector(1024)
-    USING NULL;  -- 清空現有 768 維向量（維度不可轉換）
+    IF v_dim IS DISTINCT FROM 1024 THEN
+        EXECUTE 'DROP INDEX IF EXISTS idx_rag_chunks_embedding';
+        EXECUTE $sql$
+            ALTER TABLE rag_chunks
+                ALTER COLUMN embedding TYPE vector(1024)
+                USING NULL
+        $sql$;
+
+        RAISE NOTICE 'rag_chunks.embedding migrated from vector(%) to vector(1024); old embeddings were cleared', v_dim;
+    ELSE
+        RAISE NOTICE 'rag_chunks.embedding already vector(1024); existing embeddings preserved';
+    END IF;
+END $$;
 
 -- 重建 ivfflat index（lists=100 適合 ~10k 筆以下資料）
 CREATE INDEX IF NOT EXISTS idx_rag_chunks_embedding
@@ -58,11 +98,29 @@ COMMENT ON COLUMN rag_chunks.embedding IS
 
 
 -- 3. playbook_embeddings：清空向量資料，變更欄位維度
-DROP INDEX IF EXISTS ix_playbook_embeddings_vec;
+DO $$
+DECLARE
+    v_dim integer;
+BEGIN
+    SELECT a.atttypmod INTO v_dim
+    FROM pg_attribute a
+    JOIN pg_class c ON a.attrelid = c.oid
+    WHERE c.relname = 'playbook_embeddings'
+      AND a.attname = 'embedding';
 
-ALTER TABLE playbook_embeddings
-    ALTER COLUMN embedding TYPE vector(1024)
-    USING NULL;  -- 清空現有 768 維向量
+    IF v_dim IS DISTINCT FROM 1024 THEN
+        EXECUTE 'DROP INDEX IF EXISTS ix_playbook_embeddings_vec';
+        EXECUTE $sql$
+            ALTER TABLE playbook_embeddings
+                ALTER COLUMN embedding TYPE vector(1024)
+                USING NULL
+        $sql$;
+
+        RAISE NOTICE 'playbook_embeddings.embedding migrated from vector(%) to vector(1024); old embeddings were cleared', v_dim;
+    ELSE
+        RAISE NOTICE 'playbook_embeddings.embedding already vector(1024); existing embeddings preserved';
+    END IF;
+END $$;
 
 CREATE INDEX IF NOT EXISTS ix_playbook_embeddings_vec
     ON playbook_embeddings