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>

Cargo.lock

@@ -132,9 +132,9 @@ checksum = "4288f83726785267c6f2ef073a3d83dc3f9b81464e9f99898240cced85fce35a"
 
 [[package]]
 name = "async-trait"
-version = "0.1.91"
+version = "0.1.92"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "ae36dc4177970ef04fde5178d3e2429882def40e57a451f919c098f72baa6cec"
+checksum = "82f6aeea286b8eb4dd3431a1be1b59d290ace00f5bfd8e2a159bc2a05e2c1667"
 dependencies = [
  "proc-macro2",
  "quote",
@@ -3362,6 +3362,7 @@ dependencies = [
  "anyhow",
  "arc-swap",
  "async-nats",
+ "async-trait",
  "aws-sdk-s3",
  "axum",
  "base64 0.22.1",