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/epics/fn-1.json

@@ -0,0 +1,13 @@
+{
+  "branch_name": "fn-1",
+  "created_at": "2026-01-12T22:40:52.831445Z",
+  "depends_on_epics": [],
+  "id": "fn-1",
+  "next_task": 1,
+  "plan_review_status": "unknown",
+  "plan_reviewed_at": null,
+  "spec_path": ".flow/specs/fn-1.md",
+  "status": "open",
+  "title": "Transcript Chat Assistant (POC)",
+  "updated_at": "2026-01-12T22:40:52.831630Z"
+}