feat: 支持手动切换采集帧率

在 Android、Web 及 Flutter 控制端新增帧率选择 UI，被控端按屏幕刷新率生成支持的帧率档位并上报，允许在保持分辨率不变的情况下仅切换帧率。

另附带更新信号服务器的数据库配置。

webrtc_controller_ios/web_rtc_controller_ios/WebRTC/WebRTCClient.swift

@@ -12,6 +12,8 @@ protocol WebRTCClientDelegate: AnyObject {
     func webRTCClient(didReportStreamMode mode: Int)
     /// 被控端上报当前采集分辨率
     func webRTCClient(didReportResolution width: Int, height: Int)
+    /// 被控端上报当前采集帧率与支持的帧率档位（按屏幕刷新率筛选，升序）
+    func webRTCClient(didReportFps fps: Int, supportedFps: [Int])
 }
 
 /// WebRTC 客户端：负责 PeerConnection 的创建、Offer/Answer 协商、
@@ -313,6 +315,8 @@ extension WebRTCClient: RTCDataChannelDelegate {
                 self.delegate?.webRTCClient(didReportStreamMode: Int(msg.streamMode))
             case .reportResolution:
                 self.delegate?.webRTCClient(didReportResolution: Int(msg.width), height: Int(msg.height))
+                self.delegate?.webRTCClient(didReportFps: Int(msg.fps),
+                                            supportedFps: msg.supportedFps.map(Int.init))
             default:
                 break
             }