Persist chat history to Postgres via durable JetStream consumer

DATABASE_URL required at startup; schema self-initializes.
Compose gains a postgres service for standalone dev.

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

.env.example

@@ -3,6 +3,9 @@
 # NATS server the chat server publishes/subscribes on.
 NATS_URL=nats://127.0.0.1:4222
 
+# Postgres archive for message history (written by the JetStream consumer).
+DATABASE_URL=postgres://cnats:cnats@127.0.0.1:5432/cnats
+
 # Base URL of your Kanidm instance (no trailing slash).
 KANIDM_URL=https://idm.example.com