gateway: deploy config.yaml changes (the volume was shadowing the image)

Deploying the audio-pricing change proved the gap: the image built and the app
restarted, but /model/info still served the old prices — /app/data is a
persistent volume, so the Dockerfile's COPY config.yaml /app/data/config.yaml is
shadowed by the volume's file from the first install, and start.sh only wrote a
default when the file was missing. Config changes in this repo therefore never
reached a running gateway; the audit's byte-identical md5 check was a manual
sync.

Fix: ship a second copy at /app/code/config.yaml and have start.sh sync it over
the volume at boot whenever they differ. The repo stays the source of truth, the
legacy first-boot default remains, and the README documents the flow.

start.sh

@@ -150,6 +150,17 @@ YAML
     echo "Config written to $CONFIG_FILE"
 fi
 
+# --- Ship config updates with the image ---
+# /app/data is a persistent volume, so the copy the Dockerfile bakes into
+# /app/data/config.yaml is shadowed by the volume's file forever — config
+# changes in the repo would never reach a running gateway. The repo is the
+# source of truth (README: repos mirror production), so sync the shipped copy
+# over the volume whenever they differ.
+if [ -f /app/code/config.yaml ] && ! cmp -s /app/code/config.yaml "$CONFIG_FILE"; then
+    echo "Updating /app/data/config.yaml from the image (repo is the source of truth)"
+    cp /app/code/config.yaml "$CONFIG_FILE"
+fi
+
 # --- Start LiteLLM ---
 echo "Starting LiteLLM on port 4000..."