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/go.mod

@@ -5,6 +5,7 @@ go 1.24.0
 toolchain go1.24.11
 
 require (
+	entgo.io/ent v0.14.5
 	github.com/gin-gonic/gin v1.9.1
 	github.com/golang-jwt/jwt/v5 v5.2.0
 	github.com/google/uuid v1.6.0
@@ -23,17 +24,17 @@ require (
 	golang.org/x/net v0.47.0
 	golang.org/x/term v0.37.0
 	gopkg.in/yaml.v3 v3.0.1
-	gorm.io/datatypes v1.2.0
-	gorm.io/driver/postgres v1.5.4
-	gorm.io/gorm v1.25.5
 )
 
 require (
+	ariga.io/atlas v0.32.1-0.20250325101103-175b25e1c1b9 // indirect
 	dario.cat/mergo v1.0.2 // indirect
-	filippo.io/edwards25519 v1.1.0 // indirect
 	github.com/Azure/go-ansiterm v0.0.0-20210617225240-d185dfc1b5a1 // indirect
 	github.com/Microsoft/go-winio v0.6.2 // indirect
+	github.com/agext/levenshtein v1.2.3 // indirect
 	github.com/andybalholm/brotli v1.2.0 // indirect
+	github.com/apparentlymart/go-textseg/v15 v15.0.0 // indirect
+	github.com/bmatcuk/doublestar v1.3.4 // indirect
 	github.com/bytedance/sonic v1.9.1 // indirect
 	github.com/cenkalti/backoff/v4 v4.3.0 // indirect
 	github.com/cespare/xxhash/v2 v2.3.0 // indirect
@@ -58,22 +59,19 @@ require (
 	github.com/go-logr/logr v1.4.3 // indirect
 	github.com/go-logr/stdr v1.2.2 // indirect
 	github.com/go-ole/go-ole v1.2.6 // indirect
+	github.com/go-openapi/inflect v0.19.0 // indirect
 	github.com/go-playground/locales v0.14.1 // indirect
 	github.com/go-playground/universal-translator v0.18.1 // indirect
 	github.com/go-playground/validator/v10 v10.14.0 // indirect
-	github.com/go-sql-driver/mysql v1.9.0 // indirect
 	github.com/goccy/go-json v0.10.2 // indirect
+	github.com/google/go-cmp v0.7.0 // indirect
 	github.com/google/go-querystring v1.1.0 // indirect
 	github.com/google/subcommands v1.2.0 // indirect
 	github.com/grpc-ecosystem/grpc-gateway/v2 v2.27.3 // indirect
 	github.com/hashicorp/hcl v1.0.0 // indirect
+	github.com/hashicorp/hcl/v2 v2.18.1 // indirect
 	github.com/icholy/digest v1.1.0 // indirect
-	github.com/jackc/pgpassfile v1.0.0 // indirect
-	github.com/jackc/pgservicefile v0.0.0-20240606120523-5a60cdf6a761 // indirect
-	github.com/jackc/pgx/v5 v5.7.4 // indirect
-	github.com/jackc/puddle/v2 v2.2.2 // indirect
-	github.com/jinzhu/inflection v1.0.0 // indirect
-	github.com/jinzhu/now v1.1.5 // indirect
+	github.com/inconshreveable/mousetrap v1.1.0 // indirect
 	github.com/json-iterator/go v1.1.12 // indirect
 	github.com/klauspost/compress v1.18.1 // indirect
 	github.com/klauspost/cpuid/v2 v2.2.4 // indirect
@@ -82,7 +80,9 @@ require (
 	github.com/magiconair/properties v1.8.10 // indirect
 	github.com/mattn/go-colorable v0.1.13 // indirect
 	github.com/mattn/go-isatty v0.0.20 // indirect
+	github.com/mattn/go-runewidth v0.0.15 // indirect
 	github.com/mdelapenya/tlscert v0.2.0 // indirect
+	github.com/mitchellh/go-wordwrap v1.0.1 // indirect
 	github.com/mitchellh/mapstructure v1.5.0 // indirect
 	github.com/moby/docker-image-spec v1.3.1 // indirect
 	github.com/moby/go-archive v0.1.0 // indirect
@@ -94,6 +94,7 @@ require (
 	github.com/modern-go/concurrent v0.0.0-20180306012644-bacd9c7ef1dd // indirect
 	github.com/modern-go/reflect2 v1.0.2 // indirect
 	github.com/morikuni/aec v1.0.0 // indirect
+	github.com/olekukonko/tablewriter v0.0.5 // indirect
 	github.com/opencontainers/go-digest v1.0.0 // indirect
 	github.com/opencontainers/image-spec v1.1.1 // indirect
 	github.com/pelletier/go-toml/v2 v2.2.2 // indirect
@@ -103,6 +104,7 @@ require (
 	github.com/quic-go/qpack v0.5.1 // indirect
 	github.com/quic-go/quic-go v0.56.0 // indirect
 	github.com/refraction-networking/utls v1.8.1 // indirect
+	github.com/rivo/uniseg v0.2.0 // indirect
 	github.com/sagikazarmark/locafero v0.4.0 // indirect
 	github.com/sagikazarmark/slog-shim v0.1.0 // indirect
 	github.com/shirou/gopsutil/v4 v4.25.6 // indirect
@@ -111,6 +113,7 @@ require (
 	github.com/spaolacci/murmur3 v1.1.0 // indirect
 	github.com/spf13/afero v1.11.0 // indirect
 	github.com/spf13/cast v1.6.0 // indirect
+	github.com/spf13/cobra v1.7.0 // indirect
 	github.com/spf13/pflag v1.0.5 // indirect
 	github.com/subosito/gotenv v1.6.0 // indirect
 	github.com/testcontainers/testcontainers-go v0.40.0 // indirect
@@ -121,6 +124,8 @@ require (
 	github.com/twitchyliquid64/golang-asm v0.15.1 // indirect
 	github.com/ugorji/go/codec v1.2.11 // indirect
 	github.com/yusufpapurcu/wmi v1.2.4 // indirect
+	github.com/zclconf/go-cty v1.14.4 // indirect
+	github.com/zclconf/go-cty-yaml v1.1.0 // indirect
 	go.opentelemetry.io/auto/sdk v1.1.0 // indirect
 	go.opentelemetry.io/contrib/instrumentation/net/http/otelhttp v0.49.0 // indirect
 	go.opentelemetry.io/otel v1.37.0 // indirect
@@ -137,8 +142,8 @@ require (
 	golang.org/x/sys v0.38.0 // indirect
 	golang.org/x/text v0.31.0 // indirect
 	golang.org/x/tools v0.38.0 // indirect
+	golang.org/x/tools/go/packages/packagestest v0.1.1-deprecated // indirect
 	google.golang.org/grpc v1.75.1 // indirect
 	google.golang.org/protobuf v1.36.10 // indirect
 	gopkg.in/ini.v1 v1.67.0 // indirect
-	gorm.io/driver/mysql v1.5.2 // indirect
 )