Fix y-websocket sidecar: install package before running server

npx y-websocket doesn't have a binary — need to npm install
then run node_modules/y-websocket/bin/server.cjs directly.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

docker-compose.yml

@@ -51,7 +51,8 @@ services:
     image: node:22-slim
     container_name: rnotes-yws
     restart: unless-stopped
-    command: ["npx", "y-websocket"]
+    working_dir: /app
+    command: ["sh", "-c", "npm install y-websocket && node node_modules/y-websocket/bin/server.cjs"]
     environment:
       - HOST=0.0.0.0
       - PORT=1234
@@ -67,6 +68,8 @@ services:
       - ALL
     security_opt:
       - no-new-privileges:true
+    tmpfs:
+      - /app
 
   rnotes-postgres:
     image: postgres:16-alpine