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/sqlc.yaml

@@ -0,0 +1,20 @@
+version: "2"
+sql:
+  - engine: "postgresql"
+    queries: "sql/"
+    schema: "migrations/"
+    gen:
+      go:
+        package: "queries"
+        out: "internal/repository/queries"
+        sql_package: "pgx/v5"
+        emit_json_tags: true
+        json_tags_case_style: "snake"
+        emit_interface: true
+        emit_empty_slices: true
+        emit_pointers_for_null_types: true
+        overrides:
+          - db_type: "timestamptz"
+            go_type:
+              import: "time"
+              type: "Time"