feat: add WebVTT context generation to chat WebSocket endpoint

- Import topics_to_webvtt_named and recordings controller
- Add _get_is_multitrack helper function
- Generate WebVTT context on WebSocket connection
- Add get_context message type to retrieve WebVTT
- Maintain backward compatibility with echo for other messages
- Add test fixture and test for WebVTT context generation

Implements task fn-1.2: WebVTT context generation for transcript chat

.flow/specs/fn-1.5.md

@@ -0,0 +1,102 @@
+# Task 5: Frontend WebSocket Hook
+
+**File:** `www/app/(app)/transcripts/useTranscriptChat.ts`
+**Lines:** ~60
+**Dependencies:** Task 1 (protocol defined)
+
+## Objective
+Create React hook for WebSocket chat communication.
+
+## Implementation
+```typescript
+import { useEffect, useState, useRef } from "react"
+import { WEBSOCKET_URL } from "../../lib/apiClient"
+
+type Message = {
+  id: string
+  role: "user" | "assistant"
+  text: string
+  timestamp: Date
+}
+
+export const useTranscriptChat = (transcriptId: string) => {
+  const [messages, setMessages] = useState<Message[]>([])
+  const [isStreaming, setIsStreaming] = useState(false)
+  const [currentStreamingText, setCurrentStreamingText] = useState("")
+  const wsRef = useRef<WebSocket | null>(null)
+
+  useEffect(() => {
+    const ws = new WebSocket(
+      `${WEBSOCKET_URL}/v1/transcripts/${transcriptId}/chat`
+    )
+    wsRef.current = ws
+
+    ws.onopen = () => console.log("Chat WebSocket connected")
+
+    ws.onmessage = (event) => {
+      const msg = JSON.parse(event.data)
+
+      switch (msg.type) {
+        case "token":
+          setIsStreaming(true)
+          setCurrentStreamingText((prev) => prev + msg.text)
+          break
+
+        case "done":
+          setMessages((prev) => [
+            ...prev,
+            {
+              id: Date.now().toString(),
+              role: "assistant",
+              text: currentStreamingText,
+              timestamp: new Date(),
+            },
+          ])
+          setCurrentStreamingText("")
+          setIsStreaming(false)
+          break
+
+        case "error":
+          console.error("Chat error:", msg.message)
+          setIsStreaming(false)
+          break
+      }
+    }
+
+    ws.onerror = (error) => console.error("WebSocket error:", error)
+    ws.onclose = () => console.log("Chat WebSocket closed")
+
+    return () => ws.close()
+  }, [transcriptId, currentStreamingText])
+
+  const sendMessage = (text: string) => {
+    if (!wsRef.current) return
+
+    setMessages((prev) => [
+      ...prev,
+      {
+        id: Date.now().toString(),
+        role: "user",
+        text,
+        timestamp: new Date(),
+      },
+    ])
+
+    wsRef.current.send(JSON.stringify({ type: "message", text }))
+  }
+
+  return { messages, sendMessage, isStreaming, currentStreamingText }
+}
+```
+
+## Validation
+- [ ] Hook connects to WebSocket
+- [ ] Sends messages to server
+- [ ] Receives streaming tokens
+- [ ] Accumulates tokens into messages
+- [ ] Handles done/error events
+- [ ] Closes connection on unmount
+
+## Notes
+- Test with browser console first
+- Verify message format matches backend protocol