feat(agent): session-ready webhook for SSE-driven player handshake (0.9.13)

Closes Fase 3.3b. Daemon now tells the server the moment a session's
first HLS segment + init.mp4 land on disk; the web side flips
streaming_session.ready_at = NOW(), which its SSE endpoint pushes to
subscribed players so the loading UI flips from "Preparando…" to
"Stream listo" without polling HEAD on the segment URL.

Surface:
  - New Client.MarkSessionReady(ctx, sessionId) HTTP method →
    POST /api/internal/agent/session-ready.
  - New engine.HLSSession.ReadyCount() + FromCache() accessors so the
    watcher goroutine doesn't reach into private state.
  - New cmd.watchSessionReady(ctx, client, hsess, sessionId) goroutine
    polls ReadyCount every 200 ms with a 60 s deadline + short-circuits
    for cache-HIT sessions (ready the moment StartHLSSession returns).
  - Daemon callback spawns it right after streamSrv.HLS().Register so
    the watcher's lifecycle matches the session's.

Best-effort: a transient network failure on the webhook is logged + the
goroutine exits — the player's existing HEAD-probe retry path still
discovers ready state independently. The webhook is an acceleration,
not a hard dependency.

internal/cmd/daemon.go

@@ -612,6 +612,11 @@ func runDaemonStart() error {
 				return
 			}
 			streamSrv.HLS().Register(hsess)
+			// Tell the server seg-0 is on disk as soon as it lands so the
+			// player's SSE subscription flips its "Preparando…" UI without
+			// waiting for the browser HEAD-probe loop to discover it
+			// independently. Cache-HIT sessions are ready immediately.
+			go watchSessionReady(hlsCtx, agentClient, hsess, sess.SessionID)
 		}()
 	}
 
@@ -940,3 +945,38 @@ func mirrorCORSOrigins(parent context.Context, cfg config.Config, userAgent stri
 	}
 	return out
 }
+
+// watchSessionReady polls HLSSession.ReadyCount until the first segment +
+// init.mp4 are on disk, then POSTs /api/internal/agent/session-ready so
+// the web side flips streaming_session.ready_at — which its SSE endpoint
+// pushes to subscribed players. Cache-HIT sessions are ready the moment
+// StartHLSSession returns and POST immediately.
+//
+// Bounded by a 60 s deadline so a permanently stuck encoder doesn't keep
+// a goroutine alive forever; if seg-0 never lands the player falls back
+// to its existing HEAD-probe retry path anyway.
+func watchSessionReady(ctx context.Context, client *agent.Client, hsess *engine.HLSSession, sessionID string) {
+	deadline := time.Now().Add(60 * time.Second)
+	ticker := time.NewTicker(200 * time.Millisecond)
+	defer ticker.Stop()
+	for {
+		// Cache HIT or seg-0 ready → notify + done.
+		if hsess.FromCache() || hsess.ReadyCount() >= 1 {
+			rctx, cancel := context.WithTimeout(context.Background(), 10*time.Second)
+			if err := client.MarkSessionReady(rctx, sessionID); err != nil {
+				log.Printf("[hls %s] mark-ready failed: %v", agent.ShortID(sessionID), err)
+			}
+			cancel()
+			return
+		}
+		select {
+		case <-ctx.Done():
+			return
+		case <-ticker.C:
+		}
+		if time.Now().After(deadline) {
+			log.Printf("[hls %s] mark-ready: timeout waiting for seg-0", agent.ShortID(sessionID))
+			return
+		}
+	}
+}