feat(video): 事务化切换与前端统一编排，增强视频输入格式支持

- 后端：切换事务+transition_id，/stream/mode 返回 switching/transition_id 与实际 codec

- 事件：新增 mode_switching/mode_ready，config/webrtc_ready/mode_changed 关联事务

- 编码/格式：扩展 NV21/NV16/NV24/RGB/BGR 输入与转换链路，RKMPP direct input 优化

- 前端：useVideoSession 统一切换，失败回退真实切回 MJPEG，菜单格式同步修复

- 清理：useVideoStream 降级为 MJPEG-only

web/src/composables/useVideoSession.ts

@@ -0,0 +1,185 @@
+import { ref } from 'vue'
+
+export interface StreamModeSwitchingEvent {
+  transition_id: string
+  to_mode: string
+  from_mode: string
+}
+
+export interface StreamModeReadyEvent {
+  transition_id: string
+  mode: string
+}
+
+export interface WebRTCReadyEvent {
+  codec: string
+  hardware: boolean
+  transition_id?: string
+}
+
+let singleton: ReturnType<typeof createVideoSession> | null = null
+
+function createVideoSession() {
+  const localSwitching = ref(false)
+  const backendSwitching = ref(false)
+  const activeTransitionId = ref<string | null>(null)
+  const expectedTransitionId = ref<string | null>(null)
+
+  let lastUserSwitchAt = 0
+
+  let webrtcReadyWaiter: {
+    transitionId: string
+    resolve: (ready: boolean) => void
+    timer: ReturnType<typeof setTimeout>
+  } | null = null
+
+  let modeReadyWaiter: {
+    transitionId: string
+    resolve: (mode: string | null) => void
+    timer: ReturnType<typeof setTimeout>
+  } | null = null
+
+  function startLocalSwitch() {
+    localSwitching.value = true
+  }
+
+  function tryStartLocalSwitch(minIntervalMs = 800): boolean {
+    const now = Date.now()
+    if (localSwitching.value) return false
+    if (now - lastUserSwitchAt < minIntervalMs) return false
+    lastUserSwitchAt = now
+    localSwitching.value = true
+    return true
+  }
+
+  function endLocalSwitch() {
+    localSwitching.value = false
+  }
+
+  function clearWaiters() {
+    if (webrtcReadyWaiter) {
+      clearTimeout(webrtcReadyWaiter.timer)
+      webrtcReadyWaiter.resolve(false)
+      webrtcReadyWaiter = null
+    }
+    if (modeReadyWaiter) {
+      clearTimeout(modeReadyWaiter.timer)
+      modeReadyWaiter.resolve(null)
+      modeReadyWaiter = null
+    }
+    expectedTransitionId.value = null
+  }
+
+  function registerTransition(transitionId: string) {
+    expectedTransitionId.value = transitionId
+    activeTransitionId.value = transitionId
+    backendSwitching.value = true
+  }
+
+  function isStaleTransition(transitionId?: string): boolean {
+    if (!transitionId) return false
+    return expectedTransitionId.value !== null && transitionId !== expectedTransitionId.value
+  }
+
+  function waitForWebRTCReady(transitionId: string, timeoutMs = 3000): Promise<boolean> {
+    if (webrtcReadyWaiter) {
+      clearTimeout(webrtcReadyWaiter.timer)
+      webrtcReadyWaiter.resolve(false)
+      webrtcReadyWaiter = null
+    }
+
+    return new Promise((resolve) => {
+      const timer = setTimeout(() => {
+        if (webrtcReadyWaiter?.transitionId === transitionId) {
+          webrtcReadyWaiter = null
+        }
+        resolve(false)
+      }, timeoutMs)
+
+      webrtcReadyWaiter = {
+        transitionId,
+        resolve,
+        timer,
+      }
+    })
+  }
+
+  function waitForModeReady(transitionId: string, timeoutMs = 5000): Promise<string | null> {
+    if (modeReadyWaiter) {
+      clearTimeout(modeReadyWaiter.timer)
+      modeReadyWaiter.resolve(null)
+      modeReadyWaiter = null
+    }
+
+    return new Promise((resolve) => {
+      const timer = setTimeout(() => {
+        if (modeReadyWaiter?.transitionId === transitionId) {
+          modeReadyWaiter = null
+        }
+        resolve(null)
+      }, timeoutMs)
+
+      modeReadyWaiter = {
+        transitionId,
+        resolve,
+        timer,
+      }
+    })
+  }
+
+  function onModeSwitching(data: StreamModeSwitchingEvent) {
+    if (localSwitching.value && expectedTransitionId.value && data.transition_id !== expectedTransitionId.value) {
+      return
+    }
+    backendSwitching.value = true
+    activeTransitionId.value = data.transition_id
+    expectedTransitionId.value = data.transition_id
+  }
+
+  function onModeReady(data: StreamModeReadyEvent) {
+    if (isStaleTransition(data.transition_id)) return
+
+    backendSwitching.value = false
+    activeTransitionId.value = null
+    expectedTransitionId.value = null
+
+    if (modeReadyWaiter?.transitionId === data.transition_id) {
+      clearTimeout(modeReadyWaiter.timer)
+      modeReadyWaiter.resolve(data.mode)
+      modeReadyWaiter = null
+    }
+  }
+
+  function onWebRTCReady(data: WebRTCReadyEvent) {
+    if (isStaleTransition(data.transition_id)) return
+    if (data.transition_id && webrtcReadyWaiter?.transitionId === data.transition_id) {
+      clearTimeout(webrtcReadyWaiter.timer)
+      webrtcReadyWaiter.resolve(true)
+      webrtcReadyWaiter = null
+    }
+  }
+
+  return {
+    localSwitching,
+    backendSwitching,
+    activeTransitionId,
+    expectedTransitionId,
+    startLocalSwitch,
+    tryStartLocalSwitch,
+    endLocalSwitch,
+    clearWaiters,
+    registerTransition,
+    waitForWebRTCReady,
+    waitForModeReady,
+    onModeSwitching,
+    onModeReady,
+    onWebRTCReady,
+  }
+}
+
+export function useVideoSession(): ReturnType<typeof createVideoSession> {
+  if (!singleton) {
+    singleton = createVideoSession()
+  }
+  return singleton
+}