the lockable http tarball protocol is meant to serve tarball flakes, by
emitting a stable `Link` header:

Link: <flakeref>; rel="immutable"

this patch now supports the new header in two places, on the appview, at
the `/archive/<ref>.tar.gz` endpoint:

λ nix flake metadata -v --refresh --no-write-lock-file 'http://127.0.0.1:3000/oppi.li/repo-19-01-26-08-04-14/archive/main.tar.gz'
unpacking 'http://127.0.0.1:3000/oppi.li/repo-19-01-26-08-04-14/archive/main.tar.gz' into the Git cache...
warning: not writing modified lock file of flake 'http://127.0.0.1:3000/oppi.li/repo-19-01-26-08-04-14/archive/main.tar.gz':
• Added input 'nixpkgs':
'github:nixos/nixpkgs/bde09022887110deb780067364a0818e89258968?narHash=sha256-tLj4KcRDLakrlpvboTJDKsrp6z2XLwyQ4Zmo%2Bw8KsY4%3D' (2026-01-19)
Resolved URL: http://127.0.0.1:3000/oppi.li/repo-19-01-26-08-04-14/archive/main.tar.gz
Locked URL: http://127.0.0.1:3000/did:plc:qfpnj4og54vl56wngdriaxug/repo-19-01-26-08-04-14/archive/a63d945ae97b84812e394207f3cc80f6525c2082.tar.gz?narHash=sha256-IdKT88RIWvWrgQFx6c%2BX3cC7JFene%2BQI9yo2rKSGoA4%3D
Path: /nix/store/0k9pv83f0qn5cm0qy82j51plryk7szx7-source
Fingerprint: 9512ee4857b31a76c1112f05161bda5280d8596b866c4f78986c6c01c1d2f419
Inputs:
└───nixpkgs: github:nixos/nixpkgs/bde09022887110deb780067364a0818e89258968?narHash=sha256-tLj4KcRDLakrlpvboTJDKsrp6z2XLwyQ4Zmo%2Bw8KsY4%3D (2026-01-19 00:39:23)

and on the knotserver, when using the `/xrpc/sh.tangled.repo.archive`
endpoint:

λ nix flake metadata -v --refresh --no-write-lock-file "http://localhost:5555/xrpc/sh.tangled.repo.archive?format=tar.gz&prefix=&ref=main&repo=did%3Aplc%3Aqfpnj4og54vl56wngdriaxug%2Frepo-19-01-26-08-04-14"
unpacking 'http://localhost:5555/xrpc/sh.tangled.repo.archive?format=tar.gz&prefix=&ref=main&repo=did:plc:qfpnj4og54vl56wngdriaxug/repo-19-01-26-08-04-14' into the Git cache...
warning: not writing modified lock file of flake 'http://localhost:5555/xrpc/sh.tangled.repo.archive?format=tar.gz&prefix=&ref=main&repo=did:plc:qfpnj4og54vl56wngdriaxug/repo-19-01-26-08-04-14':
• Added input 'nixpkgs':
'github:nixos/nixpkgs/bde09022887110deb780067364a0818e89258968?narHash=sha256-tLj4KcRDLakrlpvboTJDKsrp6z2XLwyQ4Zmo%2Bw8KsY4%3D' (2026-01-19)
Resolved URL: http://localhost:5555/xrpc/sh.tangled.repo.archive?format=tar.gz&prefix=&ref=main&repo=did:plc:qfpnj4og54vl56wngdriaxug/repo-19-01-26-08-04-14
Locked URL: http://localhost:5555/xrpc/sh.tangled.repo.archive?format=tar.gz&narHash=sha256-IdKT88RIWvWrgQFx6c%2BX3cC7JFene%2BQI9yo2rKSGoA4%3D&prefix=&ref=a63d945ae97b84812e394207f3cc80f6525c2082&repo=did:plc:qfpnj4og54vl56wngdriaxug/repo-19-01-26-08-04-14
Path: /nix/store/0k9pv83f0qn5cm0qy82j51plryk7szx7-source
Fingerprint: 9512ee4857b31a76c1112f05161bda5280d8596b866c4f78986c6c01c1d2f419
Inputs:
└───nixpkgs: github:nixos/nixpkgs/bde09022887110deb780067364a0818e89258968?narHash=sha256-tLj4KcRDLakrlpvboTJDKsrp6z2XLwyQ4Zmo%2Bw8KsY4%3D (2026-01-19 00:39:23)

note that the "Resolved URL" includes a hash of the commit.

Co-authored-by: Seongmin Lee <git@boltless.me>
Signed-off-by: oppiliappan <me@oppi.li>