Measure the video, not a PNG per frame

Detection now walks a browser-seekable H.264 proxy in MediaPipe's VIDEO
running mode. The PNG sequence it replaces was 112MB for 7.6 seconds at
1440x1920 and 1.1GB at the 900-frame limit; the proxy is 6MB, and landmarks
detected off decoded H.264 rather than off the PNGs moved at most 0.0033 of
frame width.

Three things had to be true for video mode to work, and each was measured
against the same footage decoded to PNGs:

/blob/<digest> answers byte ranges. Django's FileResponse does no Range
handling, and a media element handed 200 with no Accept-Ranges reports an
empty `seekable`, no-ops every currentTime write, and detects frame one
ninety times without raising.

A seek aims at the MIDDLE of its frame. Aiming at i/fps sits on a frame
boundary and landed one frame early 31 times in 91; (i + 0.5)/fps was exact
on all 91.

Timestamps are strictly increasing footage milliseconds. Video mode is a
tracker: a repeat leaves the graph in an error state every later call
re-throws, so the landmarker is discarded on failure, and passing the frame
index instead of i*1000/fps moved landmarks six times further from the
per-frame answer.

Frames are verified rather than trusted. requestVideoFrameCallback states
which frame it handed over, the walker discards any other and fails loudly
if the one it asked for never arrives — a stale presentation from the tail
of a previous seek is what produced "asked for frame 1 and it presented
frame 2" on a video whose seeks were in fact exact.

The proxy is re-encoded even when the upload is already H.264: HEVC is not
decodable everywhere, and footage identity is the proxy's digest. The JPEG
stills beside it are tracing references, outside the footage digest because
re-rendering them at another size is not different footage.

Verified end to end in a real browser against real footage: 228/228 frames
detected, a drawn roto face, 37 backend and 234 frontend tests green.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

clips/migrations/0004_footage_video_alter_footageframe_index.py

@@ -0,0 +1,24 @@
+# Generated by Django 5.2.17 on 2026-09-28 15:10
+
+import django.db.models.deletion
+from django.db import migrations, models
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('clips', '0003_source_extraction'),
+    ]
+
+    operations = [
+        migrations.AddField(
+            model_name='footage',
+            name='video',
+            field=models.ForeignKey(blank=True, help_text='the browser-safe proxy the page detects from; null on pre-proxy footage', null=True, on_delete=django.db.models.deletion.PROTECT, related_name='video_for', to='clips.blob'),
+        ),
+        migrations.AlterField(
+            model_name='footageframe',
+            name='index',
+            field=models.PositiveIntegerField(help_text="0-based; source frame index + 1 is the JPEG's name"),
+        ),
+    ]