video: probe frame rate via ffprobe and return on /video/generate

Adds frame_rate to GenerateVideoResponse so the mobile scrubber can step
at the source's real fps instead of a hardcoded 30. probe_video_stream_meta
gains a frame_rate field (avg_frame_rate preferred, r_frame_rate fallback,
nonsense values rejected) and is now pub so the handler can reuse it.
Cost is one ffprobe per /video/generate call; degrades silently to None
on probe failure.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

src/handlers/video.rs

@@ -25,7 +25,9 @@ use crate::files::is_valid_full_path;
 use crate::libraries;
 use crate::otel::{extract_context_from_request, global_tracer};
 use crate::state::AppState;
-use crate::video::actors::{GeneratePreviewClipMessage, QueueVideosMessage, VideoToQueue};
+use crate::video::actors::{
+    GeneratePreviewClipMessage, QueueVideosMessage, VideoToQueue, probe_video_stream_meta,
+};
 use crate::video::hls_paths;
 
 /// Response body for `POST /video/generate`. Clients consume
@@ -46,6 +48,11 @@ struct GenerateVideoResponse {
     /// transcode was queued; clients should retry the URL after a short
     /// delay (or rely on HLS.js's own retry policy).
     ready: bool,
+    /// Source-video frame rate in Hz, probed via ffprobe. `None` when the
+    /// probe failed or ffprobe couldn't parse either rate field — clients
+    /// fall back to their own default (typically 30) for frame stepping.
+    #[serde(skip_serializing_if = "Option::is_none")]
+    frame_rate: Option<f32>,
 }
 
 #[post("/video/generate")]
@@ -182,11 +189,19 @@ pub async fn generate_video(
         hls_paths::PLAYLIST_FILENAME
     );
 
+    // Probe the source for frame rate so the mobile scrubber can step at
+    // the right interval. Cheap (~tens of ms) and only runs once per video
+    // open. Probe failures degrade silently — clients have a fallback.
+    let frame_rate = probe_video_stream_meta(&full_path.to_string_lossy())
+        .await
+        .frame_rate;
+
     span.set_status(Status::Ok);
     HttpResponse::Ok().json(GenerateVideoResponse {
         playlist_url,
         content_hash: content_hash_str,
         ready,
+        frame_rate,
     })
 }