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,
     })
 }
 

src/video/actors.rs

@@ -122,16 +122,21 @@ pub fn generate_image_thumbnail_ffmpeg(path: &Path, destination: &Path) -> std::
 /// Video stream metadata needed to pick HLS encode settings. Populated by
 /// a single ffprobe call to avoid spawning multiple subprocesses per video.
 #[derive(Debug, Default)]
-struct VideoStreamMeta {
-    is_h264: bool,
+pub struct VideoStreamMeta {
+    pub is_h264: bool,
     /// Rotation in degrees (0/90/180/270). Checks both the legacy `rotate`
     /// stream tag and the modern display-matrix side data.
-    rotation: i32,
+    pub rotation: i32,
+    /// Frames per second. Prefers `avg_frame_rate` (handles VFR better than
+    /// `r_frame_rate`, which lies on variable-framerate sources). `None`
+    /// when ffprobe couldn't parse either field — caller picks a fallback.
+    pub frame_rate: Option<f32>,
 }
 
 /// Probe video stream metadata in one ffprobe call. Returns default (codec
-/// unknown, rotation 0) on any failure — callers fall back to transcoding.
-async fn probe_video_stream_meta(video_path: &str) -> VideoStreamMeta {
+/// unknown, rotation 0, fps None) on any failure — callers fall back to
+/// transcoding / a default framerate.
+pub async fn probe_video_stream_meta(video_path: &str) -> VideoStreamMeta {
     let output = tokio::process::Command::new("ffprobe")
         .arg("-v")
         .arg("error")
@@ -140,7 +145,7 @@ async fn probe_video_stream_meta(video_path: &str) -> VideoStreamMeta {
         .arg("-print_format")
         .arg("json")
         .arg("-show_entries")
-        .arg("stream=codec_name:stream_tags=rotate:side_data_list")
+        .arg("stream=codec_name,r_frame_rate,avg_frame_rate:stream_tags=rotate:side_data_list")
         .arg(video_path)
         .output()
         .await;
@@ -191,12 +196,41 @@ async fn probe_video_stream_meta(video_path: &str) -> VideoStreamMeta {
         })
         .unwrap_or(0);
 
+    // ffprobe reports frame rates as rational strings like "30000/1001".
+    // Prefer avg_frame_rate (handles VFR) and fall back to r_frame_rate.
+    // Reject 0/0 (unknown), negative numbers, and anything wildly out of
+    // range so a malformed probe can't poison the scrubber's step size.
+    let parse_rational = |s: &str| -> Option<f32> {
+        let (num, den) = s.split_once('/')?;
+        let num: f32 = num.parse().ok()?;
+        let den: f32 = den.parse().ok()?;
+        if den.abs() < f32::EPSILON {
+            return None;
+        }
+        let v = num / den;
+        (v.is_finite() && v > 0.0 && v < 1000.0).then_some(v)
+    };
+    let frame_rate = stream
+        .get("avg_frame_rate")
+        .and_then(|v| v.as_str())
+        .and_then(parse_rational)
+        .or_else(|| {
+            stream
+                .get("r_frame_rate")
+                .and_then(|v| v.as_str())
+                .and_then(parse_rational)
+        });
+
     debug!(
-        "Probed {}: codec_h264={}, rotation={}°",
-        video_path, is_h264, rotation
+        "Probed {}: codec_h264={}, rotation={}°, fps={:?}",
+        video_path, is_h264, rotation, frame_rate
     );
 
-    VideoStreamMeta { is_h264, rotation }
+    VideoStreamMeta {
+        is_h264,
+        rotation,
+        frame_rate,
+    }
 }
 
 /// Probe the max keyframe interval (GOP) in the first ~30s of a video.