Bump PocketBase to 0.39.8

Update the default PB_VERSION from 0.39.7 to 0.39.8 (latest release)
across .env.example, both Dockerfiles, and both compose files.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

docker-compose.root.yml

@@ -10,8 +10,8 @@ services:
       dockerfile: Dockerfile.root
       args:
         # PocketBase version to bake into the image (see .env)
-        PB_VERSION: ${PB_VERSION:-0.39.7}
-    image: pocketbase:${PB_VERSION:-0.39.7}-root
+        PB_VERSION: ${PB_VERSION:-0.39.8}
+    image: pocketbase:${PB_VERSION:-0.39.8}-root
     container_name: ${CONTAINER_NAME:-pocketbase}-root
     restart: unless-stopped
     environment: