fix(render): loop a bounded music bed so long reels render (MusicGen CPU cap)

A multi-segment reel needs music as long as the reel, but MusicGen-small stalls on
CPU past ~45s (confirmed on the 49s downhill/soccer reels). Decouple music-generation
length from reel length: generate a bounded bed (music-gen-max-seconds, default 15s)
and loop it across the timeline (-stream_loop -1 on the music input; the existing
atrim bounds it to the cue). Reel length stays uncapped; music generation stays
feasible. Verified: 7-segment 45s downhill reel now renders (14.9s bed looped,
1080x1920/24fps/yuv420p/-15.6 LUFS).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/main/java/org/example/videoclips/config/VideoClippingProperties.java

@@ -545,6 +545,13 @@ public class VideoClippingProperties {
         /** R12 cinematic motion blur (light frame blend) in styled shots. Emulates shutter-angle blur. */
         private boolean cinematicMotionBlur = true;
 
+        /**
+         * Max seconds of music to GENERATE. Local MusicGen can't reliably produce long scores on CPU (it stalls
+         * ~45s+), so a longer reel generates a bounded bed and the renderer loops it to fill the timeline. This
+         * decouples reel length from music-generation feasibility — the reel itself stays uncapped.
+         */
+        private double musicGenMaxSeconds = 15.0;
+
         /** R13 snap montage cut boundaries onto the generated score's beat grid (cut-to-the-music). */
         private boolean beatSyncEnabled = false;
 
@@ -787,6 +794,14 @@ public class VideoClippingProperties {
             this.cinematicMotionBlur = cinematicMotionBlur;
         }
 
+        public double getMusicGenMaxSeconds() {
+            return musicGenMaxSeconds;
+        }
+
+        public void setMusicGenMaxSeconds(double musicGenMaxSeconds) {
+            this.musicGenMaxSeconds = musicGenMaxSeconds;
+        }
+
         public boolean isBeatSyncEnabled() {
             return beatSyncEnabled;
         }

src/main/java/org/example/videoclips/editing/HighlightAssetPreparationService.java

@@ -26,6 +26,7 @@ public class HighlightAssetPreparationService {
     private final EditAssetProvider assetProvider;
     private final EditAssetLibrary assetLibrary;
     private final VideoClippingProperties.Editing.Assets assets;
+    private final double musicGenMaxSeconds;
     private final ObjectMapper objectMapper;
 
     public HighlightAssetPreparationService(VideoClippingProperties properties, HighlightProjectStore store,
@@ -35,6 +36,7 @@ public class HighlightAssetPreparationService {
         this.assetProvider = assetProvider;
         this.assetLibrary = assetLibrary;
         this.assets = properties.getEditing().getAssets();
+        this.musicGenMaxSeconds = properties.getEditing().getMusicGenMaxSeconds();
         this.objectMapper = objectMapper;
     }
 
@@ -61,9 +63,12 @@ public class HighlightAssetPreparationService {
             copy(Path.of(music.get().path()), target);
             resolvedAssets.add(target.toString());
         } else {
+            // Generate a BOUNDED music bed (MusicGen stalls on long scores); the renderer loops it to fill the
+            // reel. The reel length itself is unaffected.
+            double musicSeconds = Math.min(highlight.targetDurationSeconds(), musicGenMaxSeconds);
             requestFiles.add(writeRequest(requestDirectory, projectId, highlight.highlightId(), "music",
                     "music_bed", highlight.musicDirection(), musicDirectory.resolve("music.wav"),
-                    highlight.targetDurationSeconds(), false));
+                    musicSeconds, false));
         }
 
         if (!highlight.voiceover().isEmpty()) {

src/main/java/org/example/videoclips/editing/HighlightFfmpegRenderer.java

@@ -631,7 +631,9 @@ public class HighlightFfmpegRenderer {
             }
         }
         if (music != null) {
-            command.addAll(List.of("-i", music.toString()));
+            // Loop the (bounded) generated bed so it fills the whole reel; the atrim below bounds it to the cue.
+            // Lets a long reel use a short, feasible-to-generate score instead of one impractically-long score.
+            command.addAll(List.of("-stream_loop", "-1", "-i", music.toString()));
             filters.append("[").append(input).append(":a]");
             if (musicCue != null) {
                 double duration = musicCue.timelineEndSeconds() - musicCue.timelineStartSeconds();