feat: 添加用户注册登录功能，实现数据和配置的用户隔离

- 新增 users 和 user_settings 表，repositories/analyses/review_notes 添加 user_id 列
- 实现基于邮箱+密码的注册登录，密码使用 bcrypt 哈希
- 使用 gin-contrib/sessions cookie-based session 管理
- 所有仓库、分析记录、review notes 按用户隔离
- 用户设置（LLM 配置、review 参数、缓存配置）独立存储
- 新增登录/注册页面，导航栏显示用户邮箱和退出按钮
- 前端 fetch 请求统一添加 credentials: 'same-origin'
- 支持 SESSION_SECRET 环境变量配置会话密钥

handlers/middleware.go

@@ -0,0 +1,58 @@
+package handlers
+
+import (
+	"database/sql"
+	"net/http"
+
+	"github.com/gin-contrib/sessions"
+	"github.com/gin-gonic/gin"
+
+	"github.com/HoHD/PR-Helper/models"
+)
+
+// AuthRequired returns middleware that requires a logged-in user.
+// Page requests are redirected to /login; API requests get 401 JSON.
+func AuthRequired(db *sql.DB) gin.HandlerFunc {
+	return func(c *gin.Context) {
+		session := sessions.Default(c)
+		userID := session.Get("user_id")
+		if userID == nil {
+			// Check if it's an API request
+			if len(c.Request.URL.Path) > 4 && c.Request.URL.Path[:5] == "/api/" {
+				c.AbortWithStatusJSON(http.StatusUnauthorized, gin.H{"error": "unauthenticated"})
+			} else {
+				c.Redirect(http.StatusFound, "/login")
+				c.Abort()
+			}
+			return
+		}
+
+		// Load user from DB and store in context
+		var user models.User
+		err := db.QueryRow(`SELECT id, email, password_hash, created_at, updated_at FROM users WHERE id = ?`,
+			userID).Scan(&user.ID, &user.Email, &user.PasswordHash, &user.CreatedAt, &user.UpdatedAt)
+		if err != nil {
+			// User no longer exists, clear session
+			session.Clear()
+			session.Save()
+			if len(c.Request.URL.Path) > 4 && c.Request.URL.Path[:5] == "/api/" {
+				c.AbortWithStatusJSON(http.StatusUnauthorized, gin.H{"error": "unauthenticated"})
+			} else {
+				c.Redirect(http.StatusFound, "/login")
+				c.Abort()
+			}
+			return
+		}
+
+		c.Set("user", &user)
+		c.Next()
+	}
+}
+
+// GetCurrentUser extracts the authenticated user from the Gin context.
+func GetCurrentUser(c *gin.Context) *models.User {
+	if user, exists := c.Get("user"); exists {
+		return user.(*models.User)
+	}
+	return nil
+}