refactor(数据库): 迁移持久层到 Ent 并清理 GORM

将仓储层/基础设施改为 Ent + 原生 SQL 执行路径，并移除 AutoMigrate 与 GORM 依赖。
重构内容包括：
- 仓储层改用 Ent/SQL（含 usage_log/account 等复杂查询），统一错误映射
- 基础设施与 setup 初始化切换为 Ent + SQL migrations
- 集成测试与 fixtures 迁移到 Ent 事务模型
- 清理遗留 GORM 模型/依赖，补充迁移与文档说明
- 增加根目录 Makefile 便于前后端编译

测试：
- go test -tags unit ./...
- go test -tags integration ./...

backend/migrations/007_add_user_allowed_groups.sql

@@ -0,0 +1,20 @@
+-- Add user_allowed_groups join table to replace users.allowed_groups (BIGINT[]).
+-- Phase 1: create table + backfill from the legacy array column.
+
+CREATE TABLE IF NOT EXISTS user_allowed_groups (
+    user_id     BIGINT NOT NULL REFERENCES users(id) ON DELETE CASCADE,
+    group_id    BIGINT NOT NULL REFERENCES groups(id) ON DELETE CASCADE,
+    created_at  TIMESTAMPTZ NOT NULL DEFAULT NOW(),
+    PRIMARY KEY (user_id, group_id)
+);
+
+CREATE INDEX IF NOT EXISTS idx_user_allowed_groups_group_id ON user_allowed_groups(group_id);
+
+-- Backfill from the legacy users.allowed_groups array.
+INSERT INTO user_allowed_groups (user_id, group_id)
+SELECT u.id, x.group_id
+FROM users u
+CROSS JOIN LATERAL unnest(u.allowed_groups) AS x(group_id)
+JOIN groups g ON g.id = x.group_id
+WHERE u.allowed_groups IS NOT NULL
+ON CONFLICT DO NOTHING;