Let the container run with nothing writable

The server writes to one path and no other: SESSION_FILE, from sessions.ts.
Everything else it touches on disk it only reads. So a container with a
read-only root filesystem already works -- except that `VOLUME ["/data"]`
quietly undid it. Docker acts on that directive: a container started without
`-v` gets an anonymous volume mounted there anyway, writable even under
`--read-only`. It persisted nothing across a redeploy, since each new container
got a fresh empty volume, and it left an orphan behind every time one was
replaced. Deployments that want the sessions to survive already say so
themselves -- docker-compose.yml and deploy.example.sh both mount a named
volume -- so removing the line changes nothing for them.

IMMUTABLE=1 asserts that this is how the instance is running. It is checked
rather than believed: the server refuses to start if SESSION_FILE is still set,
or if the filesystem it is installed on turns out to be writable. Left
unchecked the misconfiguration is silent, because persisting sessions is
best-effort -- a read-only /data costs one warning at the first sign-in and
nothing more until the instance is replaced and everyone is signed out.

SessionBackend names what the rest of the server asks of a session store, and
`sessions` in app.ts is typed as it. Nothing changes today; SessionStore is
still the only implementation. It is there so the OAuth work is written against
the interface rather than the class, and so the interface can record which of
its methods a stateless backend could satisfy alone: create, resolve, reseal
and destroy each touch one session, while listForUser and destroyAllForUser
have to reach sessions other than the caller's. The second of those carries the
guarantee that changing a password invalidates the sessions still holding the
old one, which is why it needs a registry -- Stalwart's token registry, once
sign-in goes through OAuth.

.env.example

@@ -28,8 +28,20 @@ SESSION_TTL=43200
 SESSION_REMEMBER_TTL=2592000
 
 # Where to persist sessions so restarts don't log everyone out (optional).
+# Leave it empty to hold sessions in memory only, which is what an immutable
+# instance does -- see IMMUTABLE below.
 SESSION_FILE=./data/sessions.json
 
+# Assert that this instance is running as an immutable container: read-only
+# root filesystem, no durable state of its own. It is checked rather than
+# taken on trust -- the server refuses to start if SESSION_FILE is set, or if
+# the filesystem it is installed on turns out to be writable. Off by default.
+# Running one looks like:
+#   docker run --read-only --tmpfs /tmp -e IMMUTABLE=1 -e SESSION_FILE= ...
+# The cost today is that a restart signs everyone out, since there is nowhere
+# left to keep the sessions. Removing that cost is what the OAuth work is for.
+# IMMUTABLE=1
+
 # Upstream timeouts / limits
 UPSTREAM_TIMEOUT=30000
 MAX_UPLOAD_BYTES=52428800