Seek 5s into videos for thumbnail extraction

The 0.5s seek often landed on the initial keyframe or a fade-in,
producing blocky thumbnails. Aim ~5s in instead, with an ffprobe
midpoint fallback for clips shorter than that and a 2s → 0
retry chain if the seek still overshoots.

internal/web/thumbs.go

@@ -17,6 +17,7 @@ import (
 	"os"
 	"os/exec"
 	"path/filepath"
+	"strconv"
 	"strings"
 	"sync"
 	"time"
@@ -305,15 +306,49 @@ func buildAvifFromFFmpeg(body []byte, outPath string) error {
 	outPNG.Close()
 	defer os.Remove(pngPath)
 
-	if err := extractVideoFrame(inPath, pngPath, "0.5"); err != nil {
-		// Some clips are shorter than the seek offset; retry from frame 0.
+	// Aim ~5s in — past the typical fade-in / first-GOP blockiness while
+	// still likely within the clip. For shorter clips we fall back to the
+	// midpoint (when ffprobe is available) so we don't seek off the end.
+	seek := "5"
+	if d := probeVideoDuration(inPath); d > 0 && d < 5 {
+		seek = strconv.FormatFloat(d/2, 'f', 2, 64)
+	}
+	if err := extractVideoFrame(inPath, pngPath, seek); err != nil {
+		// Seek overshot or container's funky — try 2s, then frame 0.
+		if err := extractVideoFrame(inPath, pngPath, "2"); err != nil {
 			if err := extractVideoFrame(inPath, pngPath, "0"); err != nil {
 				return err
 			}
 		}
+	}
 	return avifEncodeFromPNG(pngPath, outPath)
 }
 
+// probeVideoDuration returns the clip length in seconds, or 0 if ffprobe is
+// missing or can't read it (image-as-video fallback, malformed containers).
+func probeVideoDuration(inPath string) float64 {
+	if _, err := exec.LookPath("ffprobe"); err != nil {
+		return 0
+	}
+	ctx, cancel := context.WithTimeout(context.Background(), ffmpegTimeout)
+	defer cancel()
+	cmd := exec.CommandContext(ctx, "ffprobe",
+		"-v", "error",
+		"-show_entries", "format=duration",
+		"-of", "default=noprint_wrappers=1:nokey=1",
+		inPath,
+	)
+	out, err := cmd.Output()
+	if err != nil {
+		return 0
+	}
+	d, err := strconv.ParseFloat(strings.TrimSpace(string(out)), 64)
+	if err != nil || d <= 0 {
+		return 0
+	}
+	return d
+}
+
 // extractVideoFrame asks ffmpeg to seek to seekSec, grab one frame, and
 // downscale it to at most thumbWidth wide before writing pngPath.
 func extractVideoFrame(inPath, pngPath, seekSec string) error {