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/cmd/server/main.go

@@ -0,0 +1,47 @@
+package main
+
+import (
+	"context"
+	"log/slog"
+	"os"
+	"os/signal"
+	"syscall"
+
+	"github.com/fish/language/backend/internal/app"
+	"github.com/fish/language/backend/internal/config"
+	"github.com/fish/language/backend/internal/repository"
+)
+
+func main() {
+	ctx := context.Background()
+
+	cfg, err := config.Load()
+	if err != nil {
+		slog.Error("failed to load config", "error", err)
+		os.Exit(1)
+	}
+
+	pool, err := repository.NewPool(ctx, cfg.DatabaseURL)
+	if err != nil {
+		slog.Error("failed to connect to database", "error", err)
+		os.Exit(1)
+	}
+	defer pool.Close()
+
+	application := app.New(cfg, pool)
+
+	go func() {
+		if err := application.Run(); err != nil {
+			slog.Error("server error", "error", err)
+			os.Exit(1)
+		}
+	}()
+
+	slog.Info("server started", "addr", cfg.HTTPAddr)
+
+	quit := make(chan os.Signal, 1)
+	signal.Notify(quit, syscall.SIGINT, syscall.SIGTERM)
+	<-quit
+
+	slog.Info("shutting down server")
+}