feat(backend): 接入 HTTP handler、router、main 装配,端到端冒烟全通

Phase 1.3b。Phase 1.3a 留的 service 现在有了 HTTP 入口,curl 能跑通完整的
"建车 → 加三次油 → 看统计 → 删记录 → 删车"全链路,核心计算命中 plan §6。

新增:
- internal/handler/common.go  通用:writeError / parseUint64 / bindJSON / sendJSON
  * 统一错误格式:{ "code": "INVALID_ARGUMENT|NOT_FOUND|...", "message": "..." }
  * parseUint64 自动把非数字 / 0 翻成 INVALID_ARGUMENT
  * bindJSON 把 gin-validator 错误也归到 INVALID_ARGUMENT
- internal/handler/errors.go   errInvalid / errInvalidVehicleID 等小工具
- internal/handler/vehicle.go  CRUD + SetDefault 六个端点
- internal/handler/refuel_record.go  CRUD + List(?vehicle_id) + Statistics
- internal/router/router.go    把所有 /api 路由挂上 Gin engine

main.go 改动:
- 引入 wire():repository → service → handler 一气串好
- /api 业务路由从 router.Build 注入
- /health 与 NoRoute 保持原状

scripts/smoke.ps1 端到端冒烟脚本(13 步):
  health → list → create vehicle → list → refuel×3(其中 1 个半箱)→
  statistics → list refuels → delete refuel → statistics 刷新 →
  delete vehicle → GET 404

核心计算命中 plan §6.3 / §19:
  320/40 = 8.0  (price_per_liter)        ✓
  256/32 = 8.0  (price_per_liter)        ✓
  32/(11000-10500)*100 = 6.4  (L/100km)  ✓
  256/500 = 0.512  (元/公里)              ✓
  半箱油(11200/10L/is_full=false)不进入标准油耗采样 ✓
  删除记录后 avg_consumption / cost_per_km 立即重算 ✓

期间踩到的两个坑(都修了):
  1) handler 用 []struct{}{} 试图给 []model.Vehicle / []model.RefuelRecord
     兜空数组,Go 强类型不接受;改用 []model.Vehicle{} 即可
  2) Statistics 列表按 refuel_time DESC 返回,正向迭代会让 mileage 算成负数
     (newer - older < 0);改成倒序迭代,prevFull 总是更老的那次

未做(留给后续):
- 单元测试(下一步 Phase 1.4 独立 commit)
- 显式 migration 文件(下次 schema 变更补;此次仍 AutoMigrate 兜底)
- Dashboard / 统计更复杂的 endpoint(基础 Statistics 已可用)
- PWA / 导入导出 / Docker(留给 Phase 1.5+)

验证:
  go vet ./...   0 issues
  go build       26 MB binary
  smoke.ps1      13/13 断言通过

fuellog-api/cmd/server/main.go

@@ -4,7 +4,7 @@
 //  1. 加载 .env
 //  2. 解析配置
 //  3. 打开 SQLite + 运行迁移
-//  4. 装配路由
+//  4. 装配 repo / service / handler / router
 //  5. 监听并优雅退出
 package main
 
@@ -23,7 +23,11 @@ import (
 
 	"github.com/kennethcheng/fuellog-api/internal/config"
 	"github.com/kennethcheng/fuellog-api/internal/db"
+	"github.com/kennethcheng/fuellog-api/internal/handler"
 	"github.com/kennethcheng/fuellog-api/internal/middleware"
+	"github.com/kennethcheng/fuellog-api/internal/repository"
+	"github.com/kennethcheng/fuellog-api/internal/router"
+	"github.com/kennethcheng/fuellog-api/internal/service"
 	"github.com/kennethcheng/fuellog-api/pkg/apperror"
 )
 
@@ -55,6 +59,7 @@ func main() {
 	slog.Info("db ready", "path", cfg.DBPath)
 
 	app := &application{cfg: cfg, db: gormDB}
+	app.wire()
 
 	srv := &http.Server{
 		Addr:              cfg.ServerAddr,
@@ -89,11 +94,22 @@ func main() {
 type application struct {
 	cfg *config.Config
 	db  *gorm.DB
-	// Phase 1.3 起注入:vehicleRepo / refuelRepo / vehicleSvc / refuelSvc
+
+	vehicleRepo repository.VehicleRepository
+	refuelRepo  repository.RefuelRecordRepository
+	vehicleSvc  service.VehicleService
+	refuelSvc   service.RefuelRecordService
+}
+
+// wire 装配 repo / service / handler。
+func (a *application) wire() {
+	a.vehicleRepo = repository.NewVehicleRepository(a.db)
+	a.refuelRepo = repository.NewRefuelRecordRepository(a.db)
+	a.vehicleSvc = service.NewVehicleService(a.vehicleRepo, a.db)
+	a.refuelSvc = service.NewRefuelRecordService(a.refuelRepo, a.vehicleRepo)
 }
 
 // router 构建并返回 Gin 引擎。
-// 当前阶段只暴露 /health;Phase 1.3 起逐步接 /api/vehicles、/api/refuels。
 func (a *application) router() *gin.Engine {
 	r := gin.New()
 	r.Use(
@@ -116,10 +132,10 @@ func (a *application) router() *gin.Engine {
 		})
 	})
 
-	// /api 路由组(Phase 1.3 起在此挂载业务路由)
-	api := r.Group("/api")
-	_ = api // 占位,避免未使用告警
-
+	router.Build(r,
+		handler.NewVehicleHandler(a.vehicleSvc),
+		handler.NewRefuelHandler(a.refuelSvc),
+	)
 	return r
 }