feat: add Docker deployment and Microsoft OAuth support

mengpost-backend/Dockerfile

@@ -0,0 +1,24 @@
+# 多阶段构建，纯 Go（CGO_ENABLED=0）+ modernc.org/sqlite，适合 Alpine运行。
+# 对外映射建议：主机 28088 ->容器 8088（见 docker-compose.yml）。
+FROM golang:alpine AS build
+WORKDIR /src
+RUN apk add --no-cache git
+COPY go.mod go.sum ./
+RUN go mod download
+COPY . .
+ENV CGO_ENABLED=0
+RUN go build -trimpath -ldflags="-s -w" -o /out/mengpost-api .
+
+FROM alpine:3.21
+RUN apk add --no-cache ca-certificates tzdata
+ENV TZ=Asia/Shanghai
+WORKDIR /app
+RUN mkdir -p /app/data
+COPY --from=build /out/mengpost-api /app/mengpost-api
+
+ENV GIN_MODE=release
+ENV MP_PORT=8088
+ENV MP_DB=/app/data/mengpost.db
+
+EXPOSE 8088
+ENTRYPOINT ["/app/mengpost-api"]