fix(migration): 使数据库迁移脚本幂等化，支持重复执行

将 008_add_co_managed_wallet_fields.up.sql 改为幂等脚本：
- 使用 DO $$ ... IF NOT EXISTS 检查列是否存在再添加
- 使用 CREATE INDEX IF NOT EXISTS 创建索引
- 使用 DROP CONSTRAINT IF EXISTS 删除约束

这确保迁移脚本可以安全地多次执行，不会因列/索引已存在而失败。

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

backend/mpc-system/migrations/008_add_co_managed_wallet_fields.up.sql

@@ -1,14 +1,24 @@
 -- Migration: 008_add_co_managed_wallet_fields
 -- Description: Add wallet_name and invite_code fields for co-managed wallet sessions
 --              and extend session_type to support 'co_managed_keygen'
+-- This migration is idempotent - safe to run multiple times
 
--- Add new columns for co-managed wallet sessions
-ALTER TABLE mpc_sessions
-    ADD COLUMN wallet_name VARCHAR(255),
-    ADD COLUMN invite_code VARCHAR(50);
+-- Add new columns for co-managed wallet sessions (idempotent)
+DO $$
+BEGIN
+    IF NOT EXISTS (SELECT 1 FROM information_schema.columns
+                   WHERE table_name = 'mpc_sessions' AND column_name = 'wallet_name') THEN
+        ALTER TABLE mpc_sessions ADD COLUMN wallet_name VARCHAR(255);
+    END IF;
 
--- Create index for invite_code lookups
-CREATE INDEX idx_mpc_sessions_invite_code ON mpc_sessions(invite_code) WHERE invite_code IS NOT NULL;
+    IF NOT EXISTS (SELECT 1 FROM information_schema.columns
+                   WHERE table_name = 'mpc_sessions' AND column_name = 'invite_code') THEN
+        ALTER TABLE mpc_sessions ADD COLUMN invite_code VARCHAR(50);
+    END IF;
+END $$;
+
+-- Create index for invite_code lookups (idempotent)
+CREATE INDEX IF NOT EXISTS idx_mpc_sessions_invite_code ON mpc_sessions(invite_code) WHERE invite_code IS NOT NULL;
 
 -- Drop the existing session_type constraint
 ALTER TABLE mpc_sessions DROP CONSTRAINT IF EXISTS chk_session_type;
@@ -17,6 +27,6 @@ ALTER TABLE mpc_sessions DROP CONSTRAINT IF EXISTS chk_session_type;
 ALTER TABLE mpc_sessions
     ADD CONSTRAINT chk_session_type CHECK (session_type IN ('keygen', 'sign', 'co_managed_keygen'));
 
--- Add comment for the new columns
+-- Add comment for the new columns (safe to run multiple times)
 COMMENT ON COLUMN mpc_sessions.wallet_name IS 'Wallet name for co-managed wallet sessions';
 COMMENT ON COLUMN mpc_sessions.invite_code IS 'Invite code for co-managed wallet sessions - used for participants to join';