Use dedicated JSON endpoint for snapshots

[chreekat]

Closes #6871

For context on the changing API, see:

• Stackage website, snapshots page > 1 serves a json data stream instead of a normal webpage stackage-server#355 (initial change)

• /snapshots api breaking change stackage-server#365 (bugfix for breaking Stack)

• Any changes that could be relevant to users have been recorded in ChangeLog.md. (none)

• The documentation has been updated, if necessary (none)

Testing

BEFORE:

$ stack -- ls snapshots remote |& head -n2
JSONParseException Request {
  host                 = "www.stackage.org"

AFTER:

$ stack run stack -- ls snapshots remote |& head -n5
a month ago

Snapshot name: nightly-2026-03-14
Stackage Nightly 2026-03-14 (ghc-9.12.3)

[chreekat]

Hey actually I can add a CI test for the subcommand.

Huh, I could also add an integration test at the stackage-server repo. Technology at work!

[chreekat]

The tests can be future work if you just want to merge this now.

Actually, it would be a bad to add a test in stack, since a fault in Stackage shouldn't break the stack contribution workflow. So I won't do that.