Make the app's port configurable via DEVPLACE_PORT

make dev/prod hardcoded uvicorn to port 10500 with no way to override it,
so config.PORT (and everything derived from it - INTERNAL_BASE_URL,
Devii's own base URL defaults) stayed 10500 regardless of what port the
process actually bound to. DEVPLACE_PORT now drives all of it, defaulting
to 10500. Docker's existing PORT var (the externally published port via
nginx) is unrelated and untouched; its app container pins DEVPLACE_PORT
to 10500 explicitly so a bare-metal .env value can never leak in and
desync it from the Dockerfile's fixed internal bind port.

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

.env.example

@@ -29,9 +29,16 @@ SECRET_KEY=change-me
 # from the request.
 DEVPLACE_SITE_URL=
 
-# Host port the nginx front door binds.
+# Host port the nginx front door binds (Docker only - the app container's own
+# internal port stays fixed).
 PORT=10500
 
+# Port the uvicorn process itself binds to for `make dev`/`make prod` (bare
+# metal, no Docker/nginx in front). Also what the app calls itself on
+# internally (DEVII_BASE_URL default, INTERNAL_GATEWAY_URL). Unrelated to
+# PORT above - leave unset unless running bare metal on a non-default port.
+# DEVPLACE_PORT=10500
+
 # nginx upload ceiling. Must be >= the admin-configurable max_upload_size_mb.
 NGINX_MAX_BODY_SIZE=50m