Keep the last few images, and record delete-all-spam as verified live

Every deploy tags an image with its version, which is what makes a
rollback a `docker run` rather than a rebuild -- and what has quietly
put 7.7 GB of them on the host, 4.3 GB of it reclaimable. `docker image
prune` will not help: they are tagged, which is the whole point of them.

The script that creates them now removes them, keeping the newest
IHASMAIL_KEEP_VERSIONS (three by default, 0 to keep everything).

Two things it is careful about, both learned from what could go wrong
rather than from something that did:

  - it runs only after the new container reports healthy, so a rollback
    target is never dropped while the thing meant to replace it is still
    unproven.

  - it excludes the image the container is actually running, by asking
    docker what that is rather than assuming it sorts newest. After a
    rollback it does not: the running image is an old one, and naive
    keep-the-newest-N would delete the image in use. Docker would refuse,
    but being refused is not the same as not having tried.

Exercised against a stubbed docker on PATH, so the pipeline and xargs
are the real ones: keeps 3 and removes the oldest, keeps 1 and removes
three, keeps everything at 0, never lists :current, and spares the
running image in a rollback where it is the fourth-newest.

Also records delete-all-spam as confirmed live on 0.16.19 today: Junk
Mail emptied and Deleted Items stayed empty afterwards, which is the
half a mock cannot prove and the half the feature exists for.

README.md

@@ -239,6 +239,14 @@ Each build is tagged with its own version as well as `:current`, so rolling
 back is running the previous tag rather than rebuilding it. The environment
 file is never read by the script, only handed to `docker run --env-file`.
 
+Those images add up — roughly 650 MB each, one per deploy, and `docker image
+prune` will not touch them because they are tagged. After a deploy reports
+healthy, the script removes all but the newest `IHASMAIL_KEEP_VERSIONS` of them
+(three by default; `0` keeps every version). It never removes the image the
+container is actually running, which matters after a rollback, when that is an
+old one — and it prunes only *after* the health check, so a rollback target is
+never dropped while the thing replacing it is still unproven.
+
 ## Configuration
 
 All configuration is via environment variables (see `.env.example`):
@@ -280,6 +288,7 @@ works the same way — and dropped where 0.15 was the whole subject. Support for
 0.15 was removed on 2026-08-26; the last release that runs on it is tagged
 [`stalwart-0.15-support`](https://github.com/LINUXexpert-org/ihasmail/releases/tag/stalwart-0.15-support).
 
+- **Delete all spam destroys, and does not pass through Deleted Items** — this is the point of the feature and the thing worth checking on a real server, since a folder that empties into another folder has solved nothing. `Email/set destroy`, walked a page at a time so it survives `maxObjectsInSet` the way emptying Deleted Items already had to. **Confirmed live on 0.16.19 (2026-08-26)**: Junk Mail emptied and Deleted Items stayed empty afterwards. There is no undo, which is why all three entry points share one dialog that says so. Only Deleted Items and Junk Mail can be emptied this way, enforced in the store rather than only hidden in the menus.
 - **Read receipts are built here, not by the server** — JMAP has an extension for them, [RFC 9007](https://www.rfc-editor.org/rfc/rfc9007.html)'s `MDN/send`, and Stalwart does not implement it: `urn:ietf:params:jmap:mdn` is not among its capabilities. So ihasmail assembles the `multipart/report` itself and sends it the long way round — raw MIME uploaded as a blob, `Email/import`, then `EmailSubmission` — which is also why the receipt lands in Sent, where it honestly belongs. Non-ASCII parts are base64 rather than `8bit`, so nothing depends on 8BITMIME surviving every hop. There is deliberately no "always send" setting: a receipt confirms to whoever asked that the address is live and when it was read, to an address of the sender's choosing, so each one is a decision. Verified against the mock end to end (upload, import, submit, `$mdnsent`), and **confirmed live on 0.16.19 (2026-08-26)**: a receipt asked for by a real sender was assembled, uploaded, imported and submitted, landed in Sent, and set `$mdnsent` so a second look does not offer to send another.
 - **Where 0.16 advertises `urn:stalwart:jmap`** — not where a JMAP client would look, and this now decides whether a sign-in is allowed at all. Stalwart builds the session-level `capabilities` from a fixed list (`Session::new`, plus WebSocket) that has never contained this capability, in any 0.16.x from 0.16.0 to 0.16.19. It hands it out per-account instead, so it appears in `primaryAccounts` and in each account's `accountCapabilities`. ihasmail tested for it in `capabilities` alone, which made every real 0.16 server read as older than 0.16 — and that one check drove three things: self-service credentials fell back to `POST /api/account/auth`, which 0.16 removed, so password changes, 2FA and app passwords all failed with "this mail server does not offer self-service credential management"; About reported the wrong generation; and Files took the older code path. It now looks in all three places, and is covered by tests on each. Worth restating plainly, because the stakes went up when 0.15 support was dropped: there is no longer a fallback path for this check to be wrong *into*. Getting it wrong now refuses every sign-in against a perfectly good server — a loud failure rather than a quiet misrouting, which is the trade the removal was making.
 - **HTML signatures** — Stalwart caps a signature at 2047 **bytes** (`value.len() < 2048` on a Rust string, so UTF-8 bytes, not characters). ihasmail compacts pasted HTML, moves images to Files and, if still too large, keeps the full signature in Files behind a short marker; other clients see a text fallback. Confirmed live on 0.15.5 (2026-08-24): oversized, non-ASCII and inline-image signatures all save, and a test message arrived intact at Gmail with the logo inline.