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/Makefile

@@ -0,0 +1,32 @@
+.PHONY: dev up down build logs migrate migrate-down sqlc test clean
+
+dev:
+	docker compose up --build
+
+up:
+	docker compose up -d
+
+down:
+	docker compose down
+
+build:
+	docker compose build
+
+logs:
+	docker compose logs -f api db
+
+migrate:
+	migrate -path migrations -database "$(DATABASE_URL)" up
+
+migrate-down:
+	migrate -path migrations -database "$(DATABASE_URL)" down 1
+
+sqlc:
+	sqlc generate
+
+test:
+	go test ./...
+
+clean:
+	docker compose down -v
+	rm -rf tmp