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>

Dockerfile.rootless

@@ -3,7 +3,7 @@
 # ---- build stage: fetch + verify + unzip PocketBase ----
 FROM alpine:3.22 AS build
 
-ARG PB_VERSION=0.39.7
+ARG PB_VERSION=0.39.8
 # provided automatically by buildx (e.g. amd64, arm64)
 ARG TARGETARCH