fix(数据库): 修复默认分组缺失与迁移锁阻塞

通过迁移补种默认 groups 记录，避免新装空分组
迁移锁改为 try lock + 重试并加入超时
写入 usage_logs 时保留 rate_multiplier=0 语义
测试: go test ./...

backend/migrations/008_seed_default_group.sql

@@ -0,0 +1,4 @@
+-- Seed a default group for fresh installs.
+INSERT INTO groups (name, description)
+SELECT 'default', 'Default group'
+WHERE NOT EXISTS (SELECT 1 FROM groups);