video: handle unknown/short durations in thumb + preview gen

`get_duration_seconds` now returns `Option<f64>` and falls back from
`format=duration` to `stream=duration`. Empty stdout no longer
parse-panics with "cannot parse float from empty string", which was
poisoning the preview-clip row with status=failed and re-queueing every
full scan (notably for GoPro LRV files). `generate_preview_clip` handles
the unknown-duration case by transcoding the whole file (capped at 10s).

`generate_video_thumbnail` seeks to ~50% of the probed duration instead
of a hardcoded `-ss 3`, with a first-frame fallback when the probe
returns nothing. Fixes the loop where short Snapchat clips (<3s) got
"missing thumbnail" logged on every scan because ffmpeg exited 0
without writing a frame, and never wrote the .unsupported sentinel
either.

Adds unit tests for `parse_ffprobe_duration` covering the empty-output,
N/A, multi-line, non-positive, and non-finite cases.

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

src/video/actors.rs

@@ -2,7 +2,7 @@ use crate::database::PreviewDao;
 use crate::is_video;
 use crate::libraries::Library;
 use crate::otel::global_tracer;
-use crate::video::ffmpeg::generate_preview_clip;
+use crate::video::ffmpeg::{generate_preview_clip, get_duration_seconds_blocking};
 use actix::prelude::*;
 use log::{debug, error, info, trace, warn};
 use opentelemetry::KeyValue;
@@ -108,6 +108,13 @@ pub async fn create_playlist(video_path: &str, playlist_file: &str) -> Result<Ch
 }
 
 pub fn generate_video_thumbnail(path: &Path, destination: &Path) -> std::io::Result<()> {
+    // Probe duration up front and seek to ~50% — gives a more
+    // representative frame than a fixed offset (skipping title cards on
+    // long videos, landing inside the clip on 1–2s Snapchat MP4s) and
+    // sidesteps the seek-past-EOF class of bug entirely. When duration
+    // probing fails (LRV files, fragmented MP4s, ffprobe missing) fall
+    // back to the first frame: ugly but reliable.
+    //
     // -vf scale + -c:v mjpeg mirrors `generate_image_thumbnail_ffmpeg`. The
     // filter chain matters as much as the scale does: without it, ffmpeg
     // hands the decoded frame straight to the mjpeg encoder, which rejects
@@ -115,10 +122,14 @@ pub fn generate_video_thumbnail(path: &Path, destination: &Path) -> std::io::Res
     // filter chain lets ffmpeg auto-insert the pix_fmt converter the
     // encoder needs, which is how the image-thumbnail path already handles
     // the same class of source.
-    let output = Command::new("ffmpeg")
-        .arg("-y")
-        .arg("-ss")
-        .arg("3")
+    let seek = get_duration_seconds_blocking(path).map(|d| format!("{:.3}", d / 2.0));
+
+    let mut cmd = Command::new("ffmpeg");
+    cmd.arg("-y");
+    if let Some(s) = &seek {
+        cmd.arg("-ss").arg(s);
+    }
+    let output = cmd
         .arg("-i")
         .arg(path)
         .arg("-vframes")
@@ -139,6 +150,18 @@ pub fn generate_video_thumbnail(path: &Path, destination: &Path) -> std::io::Res
             String::from_utf8_lossy(&output.stderr).trim()
         )));
     }
+    // ffmpeg can exit 0 without writing a frame for malformed files where
+    // the probe duration lies. Confirm a non-empty file actually landed —
+    // returning Err makes the caller write the `.unsupported` sentinel so
+    // we stop re-detecting on every scan.
+    let wrote = std::fs::metadata(destination)
+        .map(|m| m.len() > 0)
+        .unwrap_or(false);
+    if !wrote {
+        return Err(std::io::Error::other(
+            "ffmpeg exited successfully but produced no thumbnail output",
+        ));
+    }
     Ok(())
 }