Add a locked-down maven.<domain> block to the nginx template (#354)

The template's wildcard `listen 80` with no ACL was fixed in 50cc17f, but it
still only covered nexus/praxis/hexis. mavweb — the one service in the set
that serves an RCE surface (POST /tools defines argv internal/tool executes)
— had no block at all, so anyone wiring it up wrote their own, which is how
the wildcard got there the first time.

Adds a maven.kvmx.ru server with the same wg+LAN bind and allow/deny,
proxying 127.0.0.1:9201, with the WebSocket upgrade /ws needs, a 32m body
limit for push-to-talk PCM, and a 300s read timeout because an LLM turn on
the iGPU is slow.

Also records in deploy/ecosystem/docker-compose.yml that the sibling
`build:` paths pin nothing and ship the sibling working tree, with the
command to check what is about to be deployed. The stale public DNS records
(item 2) are outside the repo.

Verified: nginx -t on the template inside a minimal http{} accepts it.

deploy/ecosystem/docker-compose.yml

@@ -8,6 +8,18 @@
 #
 # Maven's own compose joins this same network (add `ecosystem` as an external
 # network there) to reach nexus:9740 / praxis:8989 / hexis:9741 directly.
+#
+# NO RELEASE PINNING (Vikunja #354): each `build:` below points at a sibling
+# WORKING TREE, so `up --build` ships whatever is checked out there, including
+# uncommitted edits. Before bringing this up, check what you are about to
+# deploy:
+#
+#   for r in nexus praxis hexis; do git -C ../../../$r status --short; \
+#     git -C ../../../$r log -1 --oneline; done
+#
+# The host nginx that fronts these is deploy/ecosystem/nginx.conf — it binds
+# the wg and LAN addresses only, with allow/deny. Keep it that way: none of
+# these containers has auth of its own.
 name: ecosystem
 
 services: