Sessions outlive the deploy that interrupted them

A MemoryStore meant every restart signed out every desk on the site
being deployed. Bearable while a desk is three people, and not the
kind of thing that gets better on its own.

They live in NATS KV now: the JetStream already under the events, the
projections, the content reload and the access questions. No new
service, no new store, and the same bucket a second instance of a site
would read if there ever is one. `create` draws a fresh id rather than
writing over a session already at that key, the bucket's `max_age`
collects what nobody comes back for, and a record's own expiry is what
answers a load, whatever the bucket still holds.

Proved the only way it can be: a store opened twice against one
JetStream, which is what two runs of the binary are.

Also fixed, while there: the integration test asserted a record's
projected state was one that `aggregates::transition` never writes -
it appends events, the desk handler updates the projection - so it
failed whenever it was actually run. It now asserts what it says it
does, that a mailed reply leaves the state alone. test.yml has been
red since 0.3.47 while publish.yml stayed green, which is how v0.4.0
went out over it.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

docs/operations.md

@@ -66,7 +66,7 @@ values.
 
 | | |
 |---|---|
-| `NATS_URL` | events, projections, content reload, access questions |
+| `NATS_URL` | events, projections, sessions, content reload, access questions |
 | `CONTENT_REPO`, `CONTENT_BRANCH` | the repo that is the site |
 | `KANIDM_URL`, `OAUTH2_CLIENT_ID`, `OAUTH2_CLIENT_SECRET` | sign-in |
 | `KANIDM_API_TOKEN` | invites and `grants:`; unset, neither happens |