Fix Docker build after FFmpeg release pruning

[carochacs]

What changed

• Replace the pruned dated BtbN FFmpeg autobuild URL with the maintained latest release assets for amd64 and arm64.
• Remove checksums tied to short-lived dated assets and document image digests as the deployment reproducibility boundary.
• Preserve the static GPL FFmpeg build and its bundled license/provenance labels.

Root cause

BtbN retains only a short rolling window of dated autobuild releases. The previously pinned autobuild-2026-07-03-13-21 URL now returns HTTP 404, preventing every clean Docker build.

Validation

• docker compose build web
• docker compose up -d --force-recreate web
• Container reports FFmpeg n7.1.5-12-g1fdbca85aa-20260801 with libvorbis enabled.
• GET /api/version succeeds after recreation.