Deploy a new build with verification and automatic rollback (#249)

internal/update applies a new build of Maven to the box she runs on and
undoes it when the new build does not come up. cmd/mavupdate is the only
trigger: a CLI the owner runs on the host.

Apply is health-check the running daemon, snapshot the deployed artifacts,
make build, make test, install, restart, health-check — and restore the
snapshot on any failure. The order is load-bearing:

  - The preflight health check refuses to update a daemon that is already
    not answering. Without a working baseline, a failed update and a box
    that was already broken are indistinguishable, and the rollback has
    nothing to prove itself against.
  - The snapshot is taken BEFORE the build, because make build writes its
    binaries into the working tree and on the docker deployment the tree
    is the install dir — snapshotting afterwards would snapshot the new
    artifacts and leave nothing to roll back to.
  - Verification is make build plus make test, before anything is
    deployed, so a broken tree costs time and nothing else. A failed
    verify also puts the tree's artifacts back, so a later restart by
    hand cannot deploy code that failed its own tests.
  - The rollback depends on nothing that just changed: byte-for-byte
    copies out of the snapshot dir, sha256-verified on the way in, and
    the same restart command. No build, no migration, no cooperation from
    the code being replaced. It also runs on an uncancellable context —
    a rollback interrupted halfway is worse than the failure that caused
    it. When the restore itself fails it says so and names the directory
    to copy back by hand rather than reporting a tidy rollback.

Off unless configured, and the refusals are code, not documentation. The
daemon does not import this package: there is no IPC method, no web route,
no timer and no act that can start an update, so nothing Maven says or
routes reaches it. Nothing fetches code — the new version is whatever the
owner pulled into the tree. The plan's release checker, auto-update
channel and in-process crash-loop supervisor are deliberately absent; a
process cannot reliably notice that it keeps dying, and restart-on-crash
belongs to compose or systemd. The database is never snapshotted or rolled
back; schema compatibility stays store.Migrate's job.

The config is refused at load without a health socket, since an update
that cannot check its own result cannot roll back, and refused when the
snapshot dir is inside the install dir, since a restore must not read from
what the install writes.

Vikunja #249

deploy/README.md

@@ -114,3 +114,49 @@ build on the target host, most likely in one of these:
   work fine over the core socket.
 - **netdata** — `mavpoll` reaches it via `host.docker.internal`; adjust if
   netdata runs elsewhere.
+
+## Updating her (`mavupdate`, Vikunja #249)
+
+Off unless configured, and there is deliberately no button for it. There is no
+IPC method, no web route, no timer and no act that starts an update — the trigger
+is a human running `mavupdate` on the host, which needs shell access, a strictly
+higher bar than the step-up passkey gate that guards `/tools`. She cannot update
+herself; she can be updated. Nothing here ever fetches code: the new version is
+whatever you pulled into the working tree yourself.
+
+Add an `update` block to `mavend.json` (mavend ignores it — only the CLI reads
+it), with paths as they exist **on the host**, not inside a container:
+
+```json
+"update": {
+  "source_dir": "/home/kami/apps/Maven",
+  "install_dir": "/home/kami/apps/Maven",
+  "snapshot_dir": "/var/lib/maven-snapshots",
+  "binaries": ["mavend", "mavweb", "mavsttd", "mavttsd", "mavwaked",
+               "mavenclient", "mavpoll", "mavcaldav", "mavmaild"],
+  "config_files": ["deploy/mavend.json"],
+  "restart_cmd": ["docker", "compose", "up", "-d", "--build"],
+  "health_socket": "/var/lib/docker/volumes/maven_sockets/_data/mavend.sock",
+  "health_timeout_sec": 120
+}
+```
+
+`snapshot_dir` must be outside `install_dir` (a restore must not read from what
+the install writes) and `health_socket` is required: an update that cannot check
+its own result cannot roll itself back, so the config is refused without one.
+
+Then:
+
+```sh
+mavupdate -config deploy/mavend.json verify        # make build + make test, deploys nothing
+mavupdate -config deploy/mavend.json apply -yes    # snapshot, verify, install, restart, health-check
+mavupdate -config deploy/mavend.json list          # what you can roll back to
+mavupdate -config deploy/mavend.json rollback -yes # restore the previous artifacts and restart
+```
+
+`apply` refuses to start if she is not already answering — otherwise a failed
+update and a box that was already broken are indistinguishable afterwards. On any
+failure after the install it restores the snapshot, restarts, and checks again;
+if that also fails it says so loudly and names the directory to copy back by hand.
+The database is never snapshotted or rolled back (see the package comment in
+`internal/update`); schema compatibility is `store.Migrate`'s job.