fix: thumbnail parser overflow, autostop-killed jobs, unified playhead

Thumbnails failed immediately on any clip past ~2 min. The one-pass select
expression joined frames with a flat a+b+c+... chain, which overflows ffmpeg's
recursive expression parser past ~120 terms ("Error while parsing expression").
Join the same terms as a balanced tree (depth O(log N)) instead — identical
selection, still one decode pass, exact per-tile frames preserved.

- generate_thumbnails management command: runs synchronously and streams each
  phase to stdout, so jobs are watchable without tailing app logs
- surface ffmpeg's stderr instead of swallowing it in CalledProcessError
- fly: min_machines_running = 1. Background thumbnail jobs outlive the HTTP
  request that triggers them, so scale-to-zero let the proxy autostop the VM
  mid-job and leave projects stuck "running"
- timeline: replace the two split playhead bars (pinned lanes vs scrolling
  tracks) with one full-height overlay, positioned in JS to track horizontal
  scroll and sit above the translucent header

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

scenes/management/commands/generate_thumbnails.py

@@ -0,0 +1,53 @@
+"""Run the thumbnailer synchronously and stream progress to the console.
+
+    python manage.py generate_thumbnails 1 2      # specific projects
+    python manage.py generate_thumbnails --all     # every project with a clip
+    python manage.py generate_thumbnails --failed   # only failed/stuck ones
+
+Unlike the upload path (which spawns a daemon thread and logs), this runs in the
+foreground so you can watch each ffmpeg pass and the final status without tailing
+app logs.
+"""
+from django.core.management.base import BaseCommand, CommandError
+
+from scenes.models import Project
+from scenes.thumbnails import generate_project_thumbnails
+
+
+class Command(BaseCommand):
+    help = "Generate thumbnails for projects, streaming progress to stdout."
+
+    def add_arguments(self, parser):
+        parser.add_argument("project_ids", nargs="*", type=int,
+                            help="Project ids to (re)generate.")
+        parser.add_argument("--all", action="store_true",
+                            help="Generate for every project that has a clip.")
+        parser.add_argument("--failed", action="store_true",
+                            help="Generate for projects whose status is failed/running/pending.")
+
+    def handle(self, *args, **opts):
+        ids = list(opts["project_ids"])
+        if opts["all"]:
+            ids += list(Project.objects.exclude(clip="").values_list("pk", flat=True))
+        if opts["failed"]:
+            ids += list(Project.objects
+                        .filter(thumbnail_status__in=["failed", "running", "pending"])
+                        .values_list("pk", flat=True))
+        ids = sorted(set(ids))
+        if not ids:
+            raise CommandError("no projects selected (pass ids, --all, or --failed)")
+
+        for pk in ids:
+            self.stdout.write(self.style.MIGRATE_HEADING(f"project {pk}"))
+            generate_project_thumbnails(pk, log=lambda msg: self.stdout.write(f"  {msg}"))
+
+        # report the resulting state so a sweep ends with a clear summary
+        self.stdout.write("")
+        for project in Project.objects.filter(pk__in=ids).order_by("pk"):
+            line = f"project {project.pk} [{project.thumbnail_status}] {project.name}"
+            style = (self.style.SUCCESS if project.thumbnail_status == "ready"
+                     else self.style.ERROR if project.thumbnail_status == "failed"
+                     else self.style.WARNING)
+            self.stdout.write(style(line))
+            if project.thumbnail_error:
+                self.stdout.write(f"    error: {project.thumbnail_error[:500]}")