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/.air.toml

@@ -0,0 +1,14 @@
+root = "."
+tmp_dir = "tmp"
+
+[build]
+cmd = "go build -o ./tmp/main ./cmd/server"
+bin = "./tmp/main"
+full_bin = "./tmp/main"
+include_ext = ["go", "mod", "sum"]
+exclude_dir = ["tmp", "vendor", "migrations"]
+delay = 1000
+stop_on_error = true
+
+[misc]
+clean_on_exit = true