新增语言学习后台服务

- 分类管理：支持分类的增删改查
- 单词管理：支持单词的增删改查，包含音标、释义、例句等字段
- 更新 README 项目说明

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

backend/docker/Dockerfile

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+FROM golang:1.25.8-alpine3.23 AS base
+WORKDIR /app
+RUN apk add --no-cache git make
+
+FROM base AS dev
+RUN go install github.com/air-verse/air@latest
+COPY go.mod go.sum ./
+RUN go mod download
+CMD ["air", "-c", ".air.toml"]
+
+FROM base AS builder
+COPY . .
+RUN go build -o /bin/server ./cmd/server
+
+FROM alpine:3.23 AS prod
+RUN apk add --no-cache ca-certificates
+COPY --from=builder /bin/server /bin/server
+EXPOSE 8080
+CMD ["/bin/server"]