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>

fly.toml

@@ -16,9 +16,11 @@ primary_region = "iad"
 [http_service]
   internal_port = 8000
   force_https = true
-  auto_stop_machines = "stop"      # scale to zero when idle
+  auto_stop_machines = "stop"      # extra machines can still scale to zero
   auto_start_machines = true       # wake on the next request
-  min_machines_running = 0
+  min_machines_running = 1         # keep one VM always up: background thumbnail
+                                   # jobs outlive any HTTP request, so scale-to-zero
+                                   # would let the proxy reap the VM mid-job.
 
 [[vm]]
   size = "shared-cpu-1x"