feat: initialize project skeleton with Go backend, database, and config

- Go module with standard library + gorilla/sessions + mysql driver
- Config: env-based configuration with .env file support
- Database: MySQL connection with auto-migration for 6 tables
- Seed data: 12 system tags with options, 8 system snippets
- Auth: session cookie middleware
- Handlers: auth, tags, snippets, builder, claude, dashboard, suggestions, settings
- LLM: OpenAI-compatible client with 30s timeout
- Docker: Dockerfile + docker-compose.yml
- .env.example with all configuration options

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

internal/auth/middleware.go

@@ -0,0 +1,46 @@
+package auth
+
+import (
+	"crypto/rand"
+	"encoding/hex"
+	"net/http"
+	"sync"
+
+	"github.com/gorilla/sessions"
+)
+
+var (
+	Store  *sessions.CookieStore
+	mu     sync.Mutex
+)
+
+func Init(secret string) {
+	Store = sessions.NewCookieStore([]byte(secret))
+	Store.Options = &sessions.Options{
+		Path:     "/",
+		MaxAge:   86400 * 7, // 7 days
+		HttpOnly: true,
+		SameSite: http.SameSiteLaxMode,
+	}
+}
+
+func GenerateToken() string {
+	b := make([]byte, 32)
+	rand.Read(b)
+	return hex.EncodeToString(b)
+}
+
+// AuthMiddleware checks if the user is authenticated
+func AuthMiddleware(next http.Handler) http.Handler {
+	return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		session, _ := Store.Get(r, "session")
+		auth, ok := session.Values["authenticated"].(bool)
+		if !ok || !auth {
+			w.Header().Set("Content-Type", "application/json")
+			w.WriteHeader(http.StatusUnauthorized)
+			w.Write([]byte(`{"code":401,"message":"未登录"}`))
+			return
+		}
+		next.ServeHTTP(w, r)
+	})
+}