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/.env.example

@@ -0,0 +1,20 @@
+# FuelLog API 配置文件示例
+# 复制到 .env 后按需修改;未设置的字段走默认值
+
+# HTTP 监听地址
+SERVER_ADDR=:8080
+
+# 运行环境:debug | release | test
+GIN_MODE=debug
+
+# SQLite 数据库文件路径(开发期默认 ./data/fuellog.db,容器内挂到 /data/fuellog.db)
+DB_PATH=./data/fuellog.db
+
+# 时区(影响时间格式化;内部统一 UTC 存库)
+TZ=Asia/Shanghai
+
+# 日志级别:debug | info | warn | error
+LOG_LEVEL=info
+
+# 跨域来源(逗号分隔;开发期可用 *)
+CORS_ORIGINS=*