media: H.264 fallback beside every AV1, and the post links the H.264

The AV1-only publish lasted four hours in the wild: the first viewer on
mobile Safari got "bad media error" from the raw file, because a post's
link is fetched raw — Lemmy apps and browsers play that exact URL, with
no <source> negotiation in front of it. The previous commit's note
("--raw ... if that audience matters") had it backwards: the audience
that cannot play AV1 is not a per-post judgement call, it is whoever
happens to open the thread on an iPhone.

So publish-media.sh now emits two encodings and points the post at the
compatible one:

  * Every video transcode also produces <hash>.h264.mp4 (x264 crf 23,
    same denoise, same frames), uploaded beside the AV1 under the AV1's
    hash — the poster's sibling-naming trick, reused, so fetch-media.sh
    finds it by name with nothing to look up.
  * The printed URL to paste into the post is the H.264 one. pict-rs
    can thumbnail it too, so instance thumbnails come back as a bonus.

fetch-media.sh adopting an own-origin .h264.mp4 URL swaps the AV1 back
in as the page's primary when it is on the mount, and records the H.264
as `fallback` in posts.json. The renderer turns a non-empty fallback
into a <source> pair: the AV1 first with an explicit codecs parameter —
both files are video/mp4, so the parameter is the only thing that lets
a non-AV1 browser skip to the file it can play — and the H.264 second.
Browsers with AV1 keep downloading the small file; Safari before 17
gets one that plays instead of an element that will not.

e2e gains the matching conditional gate: a page offering an av01
<source> must offer an .h264.mp4 one, so an AV1 video can never again
ship without its fallback.

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

tools/e2e.sh

@@ -383,6 +383,18 @@ if curl -s "$BASE/posts" | grep -qE '<video class="post-media__item"[^>]*poster=
 else
     bad "video poster" "no video has a poster; a black box shows until play"
 fi
+# A video offering an AV1 <source> must offer an H.264 one after it: the codecs
+# parameter is what lets a browser without AV1 (Safari before 17, Apple hardware
+# without the decoder) skip to a file it can play, and an AV1 source alone is
+# exactly the "element that will not play" the fallback pipeline exists to
+# prevent. Conditional — a build whose posts carry no AV1 has nothing to check.
+if curl -s "$BASE/posts" | grep -q 'codecs=av01'; then
+    if curl -s "$BASE/posts" | grep -qE '<source src="/media/[^"]*\.h264\.mp4" type="video/mp4">'; then
+        ok "AV1 videos carry an H.264 fallback source"
+    else
+        bad "video fallback" "an av01 <source> has no h264 sibling"
+    fi
+fi
 # preload="metadata", not auto: several 5 MB recordings must not all download on
 # page load.
 if curl -s "$BASE/posts" | grep -q 'preload="metadata"'; then