refactor backend packages and global db service

backend/internal/logic/ai/sessions.go

@@ -0,0 +1,24 @@
+package ai
+
+import (
+	"errors"
+	"strings"
+
+	"senlinai-agent/backend/internal/models"
+)
+
+type SessionService struct {
+}
+
+func NewSessionService() *SessionService {
+	return &SessionService{}
+}
+
+func (s *SessionService) Create(projectID uint, userID uint, title string) (*models.SenlinAgentAISession, error) {
+	title = strings.TrimSpace(title)
+	if title == "" {
+		return nil, errors.New("session title is required")
+	}
+	session := &models.SenlinAgentAISession{ProjectID: projectID, CreatedBy: userID, Title: title}
+	return session, models.DBService.Create(session).Error
+}