TASK 5 items 1-3: reusable stream, health banner, docs refresh

- Reusable stream: GetOrCreateReusableStreamAsync, cached via LayoutStore, bound at broadcast insert
- Health monitoring: GetStreamHealthAsync polls liveStreams.status, report-by-exception banner
- Go-live order: ensure stream → create broadcast → then start pump (URL must exist first)
- Full broadcast form scoped (TASK 5 items 6-7) in TASKS.md + ai.md
- TASK 23 trimmed to technical scope, TASK 26 added (marcom/launch kit)
- Confidential files: MONETIZATION.md + MARCOM.md (gitignored, not committed)
- .gitignore updated for confidential business files
- HANDOFF.md updated with session state and next steps

Models/YouTube.cs

@@ -10,6 +10,14 @@ public class YouTubeChannel
     public DateTime TokenExpiry { get; set; }
 }
 
+/// <summary>The channel's reusable live stream (TASK 5): created once, cached,
+/// rebound to every broadcast. The RTMP URL the encoder pushes to is the
+/// ingestion address + "/" + stream name.</summary>
+public record ReusableStream(string Id, string IngestionAddress, string StreamName)
+{
+    public string RtmpUrl => $"{IngestionAddress}/{StreamName}";
+}
+
 public class ChatMessage
 {
     public string Id { get; init; } = Guid.NewGuid().ToString();