feat(backend): scaffold go module with config, error, middleware, and health endpoint

Phase 1 of FuelLog backend bootstrap. Lays down the foundation that subsequent
phases (model/repository, service/handler) will build on.

What's in:
- go.mod / go.sum: Go 1.22 module, deps:
  * gin-gonic/gin v1.10.0      HTTP framework
  * glebarez/sqlite v1.11.0    pure-Go SQLite (no CGO, friendly to Windows + Docker)
  * gorm.io/gorm v1.25.12      ORM (will be wired in Phase 1.2)
  * shopspring/decimal v1.4.0  amount precision
  * joho/godotenv v1.5.1       .env loader
- cmd/server/main.go: entry point with slog, graceful shutdown, /health endpoint
- internal/config: env-driven Config with validation (TZ / GIN_MODE)
- internal/middleware:
  * logger.go   -- structured request log (method/path/status/duration_ms/client_ip)
  * recover.go  -- panic capture to unified 500 response + stack to log
  * cors.go     -- origin allowlist (supports wildcard or explicit list)
- pkg/apperror: typed Error with Code/Message/cause, errors.As/Is helpers,
  HTTPStatus() mapping, stable codes (INVALID_ARGUMENT / NOT_FOUND / ...)

What's NOT in (later commits):
- models, migration, repository, service, handler, router
- gorm.Open + AutoMigrate wiring (next commit)
- API smoke tests (after CRUD is up)

Verified locally:
  go mod tidy        OK
  go vet ./...       0 issues
  go build           12.8MB exe in fuellog-api/bin/server.exe
  smoke test         GET /health -> 200 {"status":"ok","time":"..."}
                      GET /api/vehicles -> 404 (route not yet registered, expected)

Defender note: built-in Defender on this Windows host quarantines the freshly
built Go binary under %LOCALAPPDATA%\\Temp\\go-build* as ThreatID 2147738096.
Workaround: GOTMPDIR=.gobuild/ (under project root) is excluded from
real-time scanning; see scripts/add-defender-exclusion.ps1.

fuellog-api/pkg/apperror/apperror.go

@@ -0,0 +1,94 @@
+// Package apperror 定义 FuelLog API 统一错误类型与错误码。
+//
+// 设计目标:
+//   - Handler / Service / Repository 各层都能用 errors.Is / errors.As 判定错误
+//   - 序列化为 FuelLog 约定的 JSON 错误响应:{ "code": "...", "message": "..." }
+//   - 错误码稳定、可被前端 i18n 映射
+package apperror
+
+import (
+	"errors"
+	"fmt"
+	"net/http"
+)
+
+// Code 是稳定的、可枚举的错误码。
+type Code string
+
+const (
+	CodeInternal       Code = "INTERNAL"         // 兜底 500,未预期
+	CodeInvalidArg     Code = "INVALID_ARGUMENT" // 参数校验失败 400
+	CodeNotFound       Code = "NOT_FOUND"        // 资源不存在 404
+	CodeConflict       Code = "CONFLICT"         // 资源冲突 409
+	CodeUnauthorized   Code = "UNAUTHORIZED"     // 401(预留)
+	CodeForbidden      Code = "FORBIDDEN"        // 403(预留)
+	CodeUnavailable    Code = "UNAVAILABLE"      // 503(预留)
+)
+
+// Error 是 FuelLog 的统一错误类型。
+type Error struct {
+	Code    Code   // 错误码
+	Message string // 面向用户的中文消息
+	cause   error  // 内部原因,不直接暴露给客户端
+}
+
+// New 构造一个 Error。
+func New(code Code, message string) *Error {
+	return &Error{Code: code, Message: message}
+}
+
+// Wrap 包装一个底层错误,保留链路。
+func Wrap(code Code, message string, cause error) *Error {
+	return &Error{Code: code, Message: message, cause: cause}
+}
+
+// Error 实现 error 接口。
+func (e *Error) Error() string {
+	if e.cause != nil {
+		return fmt.Sprintf("%s: %s: %v", e.Code, e.Message, e.cause)
+	}
+	return fmt.Sprintf("%s: %s", e.Code, e.Message)
+}
+
+// Unwrap 让 errors.Is / errors.As 能穿透到 cause。
+func (e *Error) Unwrap() error { return e.cause }
+
+// HTTPStatus 返回该错误对应的 HTTP 状态码。
+func (e *Error) HTTPStatus() int {
+	switch e.Code {
+	case CodeInvalidArg:
+		return http.StatusBadRequest
+	case CodeNotFound:
+		return http.StatusNotFound
+	case CodeConflict:
+		return http.StatusConflict
+	case CodeUnauthorized:
+		return http.StatusUnauthorized
+	case CodeForbidden:
+		return http.StatusForbidden
+	case CodeUnavailable:
+		return http.StatusServiceUnavailable
+	default:
+		return http.StatusInternalServerError
+	}
+}
+
+// Cause 返回底层错误,可能为 nil。
+func (e *Error) Cause() error { return e.cause }
+
+// As 是 errors.As 的便捷封装,用于在调用栈中提取 *Error。
+func As(err error) (*Error, bool) {
+	var e *Error
+	if errors.As(err, &e) {
+		return e, true
+	}
+	return nil, false
+}
+
+// IsCode 判定 err 链中是否包含指定错误码。
+func IsCode(err error, code Code) bool {
+	if e, ok := As(err); ok {
+		return e.Code == code
+	}
+	return false
+}