fix(identity-service): 设置 user_id 自增序列从 10 开始

- 添加数据库 migration 设置 user_id 序列起始值为 10
- 保留 user_id 1-9 给系统账户使用
- 修复用户注册时的唯一约束冲突错误
- 序列值安全检查：仅在当前值 < 10 时重置

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

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

backend/services/identity-service/prisma/migrations/20251220000000_set_user_id_sequence_start/migration.sql

@@ -0,0 +1,21 @@
+-- Set user_id sequence to start from 10
+-- This reserves user IDs 1-9 for system accounts
+
+-- Check current sequence value
+DO $$
+DECLARE
+    current_val BIGINT;
+BEGIN
+    -- Get current sequence value
+    SELECT last_value INTO current_val FROM user_accounts_user_id_seq;
+
+    -- Only reset if current value is less than 10
+    IF current_val < 10 THEN
+        -- Set sequence to start from 10
+        -- Using setval with false means next nextval() will return 10
+        PERFORM setval('user_accounts_user_id_seq', 10, false);
+        RAISE NOTICE 'Sequence reset to start from 10';
+    ELSE
+        RAISE NOTICE 'Sequence already at % (>= 10), no reset needed', current_val;
+    END IF;
+END $$;