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/005_schema_parity.sql

@@ -0,0 +1,42 @@
+-- Align SQL migrations with current GORM persistence models.
+-- This file is designed to be safe on both fresh installs and existing databases.
+
+-- users: add fields added after initial migration
+ALTER TABLE users ADD COLUMN IF NOT EXISTS username VARCHAR(100) NOT NULL DEFAULT '';
+ALTER TABLE users ADD COLUMN IF NOT EXISTS wechat VARCHAR(100) NOT NULL DEFAULT '';
+ALTER TABLE users ADD COLUMN IF NOT EXISTS notes TEXT NOT NULL DEFAULT '';
+
+-- api_keys: allow longer keys (GORM model uses size:128)
+ALTER TABLE api_keys ALTER COLUMN key TYPE VARCHAR(128);
+
+-- accounts: scheduling and rate-limit fields used by repository queries
+ALTER TABLE accounts ADD COLUMN IF NOT EXISTS schedulable BOOLEAN NOT NULL DEFAULT TRUE;
+ALTER TABLE accounts ADD COLUMN IF NOT EXISTS rate_limited_at TIMESTAMPTZ;
+ALTER TABLE accounts ADD COLUMN IF NOT EXISTS rate_limit_reset_at TIMESTAMPTZ;
+ALTER TABLE accounts ADD COLUMN IF NOT EXISTS overload_until TIMESTAMPTZ;
+ALTER TABLE accounts ADD COLUMN IF NOT EXISTS session_window_start TIMESTAMPTZ;
+ALTER TABLE accounts ADD COLUMN IF NOT EXISTS session_window_end TIMESTAMPTZ;
+ALTER TABLE accounts ADD COLUMN IF NOT EXISTS session_window_status VARCHAR(20);
+
+CREATE INDEX IF NOT EXISTS idx_accounts_schedulable ON accounts(schedulable);
+CREATE INDEX IF NOT EXISTS idx_accounts_rate_limited_at ON accounts(rate_limited_at);
+CREATE INDEX IF NOT EXISTS idx_accounts_rate_limit_reset_at ON accounts(rate_limit_reset_at);
+CREATE INDEX IF NOT EXISTS idx_accounts_overload_until ON accounts(overload_until);
+
+-- redeem_codes: subscription redeem fields
+ALTER TABLE redeem_codes ADD COLUMN IF NOT EXISTS group_id BIGINT REFERENCES groups(id) ON DELETE SET NULL;
+ALTER TABLE redeem_codes ADD COLUMN IF NOT EXISTS validity_days INT NOT NULL DEFAULT 30;
+CREATE INDEX IF NOT EXISTS idx_redeem_codes_group_id ON redeem_codes(group_id);
+
+-- usage_logs: billing type used by filters and stats
+ALTER TABLE usage_logs ADD COLUMN IF NOT EXISTS billing_type SMALLINT NOT NULL DEFAULT 0;
+CREATE INDEX IF NOT EXISTS idx_usage_logs_billing_type ON usage_logs(billing_type);
+
+-- settings: key-value store
+CREATE TABLE IF NOT EXISTS settings (
+    id          BIGSERIAL PRIMARY KEY,
+    key         VARCHAR(100) NOT NULL UNIQUE,
+    value       TEXT NOT NULL,
+    updated_at  TIMESTAMPTZ NOT NULL DEFAULT NOW()
+);
+