build(compose): run npm install in container, persist deps in named volumes

Replace the host-side `make npm-install` step with a compose-level
`command:` override that runs `npm install` inside the container on
every start. Back node_modules and static/vendor with named volumes so
the bind-mounted source tree no longer shadows the install while state
still survives container restarts. Drop the now-redundant npm-install
target and its references in up/dev/prod/test-e2e.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

docker-compose.yml

@@ -12,4 +12,13 @@ services:
       - .env
     volumes:
       - ./app:/app
+      - node_modules:/app/node_modules
+      - vendor:/app/static/vendor
+    # Run `npm install` on every container start so the named volumes
+    # reflect the current package.json (postinstall regenerates vendor/).
+    command: sh -c "npm install --prefix /app --no-audit --no-fund && python app.py"
     restart: unless-stopped
+
+volumes:
+  node_modules:
+  vendor: