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.

CHANGELOG.md

@@ -5,6 +5,21 @@ All notable changes to this project will be documented in this file.
 The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.1.0/),
 and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0.html).
 
+## [0.9.13] - 2026-05-27
+
+### Added
+
+- **Session-ready webhook** (`/api/internal/agent/session-ready`). Daemon
+  watches every new HLSSession's segment counter and, the moment seg-0 +
+  init.mp4 land on disk, POSTs the sessionId to the server. The web side
+  flips `streaming_session.ready_at = NOW()`, which its new SSE endpoint
+  pushes to subscribed players so the "Preparando…" UI flips to
+  "Stream listo" without waiting for the player's HEAD-probe retry loop
+  to discover it. Cache-HIT sessions fire the webhook immediately on
+  StartHLSSession return.
+- `engine.HLSSession.ReadyCount()` + `FromCache()` accessors so the
+  ready-watcher goroutine doesn't reach into private state.
+
 ## [0.9.12] - 2026-05-27
 
 ### Added

internal/agent/client.go

@@ -109,6 +109,27 @@ func (c *Client) ReportUpgradeResult(ctx context.Context, agentID string, succes
 	return nil
 }
 
+// MarkSessionReady signals the server that the first HLS segment + init.mp4
+// landed on disk for the given session. The web side flips
+// streaming_session.ready_at = NOW(), which its SSE endpoint emits to
+// subscribed players so the "Preparando…" UI ends without polling HEAD
+// on /hls/<id>/master.m3u8.
+//
+// Best-effort: the server is the source of truth for session state and
+// will reach the same conclusion via HEAD probes anyway if this call
+// fails. We log the error in the caller but don't retry — by the time
+// a retry would land the user is likely already playing.
+func (c *Client) MarkSessionReady(ctx context.Context, sessionID string) error {
+	req := struct {
+		SessionID string `json:"sessionId"`
+	}{SessionID: sessionID}
+	var resp StatusResponse
+	if err := c.doPost(ctx, "/api/internal/agent/session-ready", req, &resp); err != nil {
+		return fmt.Errorf("mark session ready: %w", err)
+	}
+	return nil
+}
+
 // ReportStatus reports download progress. Returns server-side flags the CLI must act on.
 func (c *Client) ReportStatus(ctx context.Context, update StatusUpdate) (*StatusResponse, error) {
 	var resp StatusResponse

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
+		}
+	}
+}

internal/cmd/version.go

@@ -1,4 +1,4 @@
 package cmd
 
 // Version is the CLI version. Overridden by goreleaser ldflags at release time.
-var Version = "0.9.12"
+var Version = "0.9.13"

internal/engine/hls.go

@@ -519,6 +519,21 @@ func (s *HLSSession) ProbeInfo() map[string]any {
 	}
 }
 
+// ReadyCount returns how many segments are currently fully on disk.
+// Caller can `>= 1` it to check whether seg-0 has landed (and so the
+// player can be told to attach). For cache-HIT sessions this is always
+// `segmentCount` from the moment StartHLSSession returns.
+func (s *HLSSession) ReadyCount() int {
+	s.readyMu.Lock()
+	defer s.readyMu.Unlock()
+	return s.readyMax
+}
+
+// FromCache reports whether this session was served from the HLS cache
+// (no ffmpeg subprocess spawned). Used by ready-watcher logic to short-
+// circuit polling — a cache HIT is ready the moment we return.
+func (s *HLSSession) FromCache() bool { return s.fromCache }
+
 // MasterPlaylist returns the rendered master.m3u8 contents.
 func (s *HLSSession) MasterPlaylist() string { return s.manifestRoot }