fix: bound the token cache, and make peer message ids unique

Two bugs from review, the second in both agents because identity.py is a copy.

The exchanged-token cache grew for the life of the process. It is keyed on
(subject token, scope) and the subject token is minted fresh every turn, so
every turn added keys nothing would ever read again — and expiry was checked on
read but never collected. A slow leak that only shows in a pod up for days,
which is the worst way to find one.

It now drops expired entries first and evicts oldest beyond a bound. Expired
first on purpose: an expired entry is dead weight wherever it sits in the
ordering, so evicting by age alone would keep it while discarding a live one.

The docstring was wrong before the code was: it claimed the cache lasted one
turn. It never did. Corrected rather than quietly made true.

message_id reused the context id, which is the conversation — and a conversation
makes several calls to the same peer, relaying an approval being exactly that.
A peer treating the second call as a replay of the first would surface as a
stale answer rather than an error.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

k8s/deployment.yaml

@@ -30,7 +30,7 @@ spec:
           # repo is public, so the pull needs no credentials — which removes an
           # imagePullSecret that would otherwise have to be created by hand in
           # this namespace and kept in step with a GitHub token.
-          image: gitea.sttlab.eu/stt/classic-models-agent-catalogue:v0.3.1
+          image: gitea.sttlab.eu/stt/classic-models-agent-catalogue:v0.3.2
           ports:
             - name: http
               containerPort: 8080