Bump PocketBase to 0.40.0

Also update the README version references, which were still on 0.39.9.

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

docker-compose.rootless.yml

@@ -10,8 +10,8 @@ services:
       dockerfile: Dockerfile.rootless
       args:
         # PocketBase version to bake into the image (see .env)
-        PB_VERSION: ${PB_VERSION:-0.39.10}
-    image: pocketbase:${PB_VERSION:-0.39.10}
+        PB_VERSION: ${PB_VERSION:-0.40.0}
+    image: pocketbase:${PB_VERSION:-0.40.0}
     container_name: ${CONTAINER_NAME:-pocketbase}
     restart: unless-stopped
     environment: