/owner/repo/archive/ref.tar.gz returns 404 on the new appview and lockable tarball support regressed #4611

Every archive URL on tangled.org now returns 404, e.g.:

• https://tangled.org/tangled.org/core/archive/master
• https://tangled.org/oppi.li/statix/archive/main.tar.gz

Existing flake.lock entries are affected too: https://tangled.org/did:plc:j5hmlfdrwkvtxm7cjmu7j2is/archive/<sha>.tar.gz now 307s to /tangled.org/core/archive/<sha>.tar.gz, which 404s.

The UI's download button now uses https://api.tangled.org/xrpc/org.tangled.temp.git.getArchive?repo=<did>&ref=<ref>&format=tar.gz&prefix=<prefix>, but:

1. it doesn't emit Link: <...>; rel="immutable", so Nix locks ref=trunk + narHash instead of a commit
2. prefix is required
3. it isn't a stable, human-writable URL (needs the repo DID, no .tar.gz suffix), so it doesn't work as a drop-in inputs.foo.url

Related: #231