fix(deploy): 修复 Docker 部署两个问题

1. 后端 Dockerfile 开启 CGO（go-sqlite3 依赖），安装 gcc/musl-dev
2. nginx 添加 /auth/ 反向代理，修复注册登录 405 错误

backend/Dockerfile

@@ -2,13 +2,13 @@
 FROM golang:1.26-alpine AS builder
 
 RUN sed -i 's/dl-cdn.alpinelinux.org/mirrors.aliyun.com/g' /etc/apk/repositories
-RUN apk add --no-cache git
+RUN apk add --no-cache git gcc musl-dev
 WORKDIR /src
 COPY go.mod go.sum ./
 RUN go env -w GOPROXY=https://goproxy.cn,direct && go mod download
 
 COPY . .
-RUN CGO_ENABLED=0 GOOS=linux go build -o /bin/gen2d ./cmd/main.go
+RUN CGO_ENABLED=1 GOOS=linux go build -o /bin/gen2d ./cmd/main.go
 
 # ---- Runtime Stage ----
 FROM alpine:3.20