Everything after this requires Stalwart 0.16 or newer. The dual-generation
support is being removed deliberately, not by neglect: 0.15 and 0.16 differ
enough -- a REST management API replaced by JMAP registry objects, a changed
FileNode shape, split rights, configuration moved into the store -- that
carrying both meant 34 branch points across nine files, a 92-line
compatibility shim whose only job was telling them apart, a parallel REST
implementation of every credential operation, and a mock that had to model
both generations.

The cost was not the code. It was that a wrong guess about which generation
answered had somewhere to fall back to, so it failed quietly instead of
loudly: a capability looked for in the wrong place downgraded every real 0.16
server to the 0.15 path, and that reached production.

What this commit supports on 0.15.5, confirmed live on 2026-08-24: the mail
flows, self-service credentials over the REST path, Files (upload, folder
creation, listing, rename, move, delete), and signatures including the
oversized and inline-image cases.

If you are on 0.15 and cannot upgrade, build from this tag. Upgrading is the
better answer, and stalwart-migrator exists for it:
https://github.com/LINUXexpert-org/stalwart-migrator