feat(migrations): 改进校验和错误提示和文档

- 增强迁移校验和不匹配的错误信息，提供具体解决方案
- 添加 migrations/README.md 文档说明迁移最佳实践
- 明确迁移不可变原则和正确的修改流程

backend/migrations/017_add_gemini_tier_id.sql

@@ -26,10 +26,5 @@ UPDATE accounts
 SET credentials = credentials - 'tier_id'
 WHERE platform = 'gemini'
   AND type = 'oauth'
-  AND jsonb_typeof(credentials) = 'object'
-  AND credentials->>'tier_id' = 'LEGACY'
-  AND (
-    credentials->>'oauth_type' = 'code_assist'
-    OR (credentials->>'oauth_type' IS NULL AND credentials->>'project_id' IS NOT NULL)
-  );
+  AND credentials->>'oauth_type' = 'code_assist';
 -- +goose StatementEnd