feat(groups): add Claude Code client restriction and session isolation

- Add claude_code_only field to restrict groups to Claude Code clients only
- Add fallback_group_id for non-Claude Code requests to use alternate group
- Implement ClaudeCodeValidator for User-Agent detection
- Add group-level session binding isolation (groupID in Redis key)
- Prevent cross-group sticky session pollution
- Update frontend with Claude Code restriction controls

backend/internal/handler/openai_gateway_handler.go

@@ -206,7 +206,7 @@ func (h *OpenAIGatewayHandler) Responses(c *gin.Context) {
 				h.handleConcurrencyError(c, err, "account", streamStarted)
 				return
 			}
-			if err := h.gatewayService.BindStickySession(c.Request.Context(), sessionHash, account.ID); err != nil {
+			if err := h.gatewayService.BindStickySession(c.Request.Context(), apiKey.GroupID, sessionHash, account.ID); err != nil {
 				log.Printf("Bind sticky session failed: %v", err)
 			}
 		}