Bump PocketBase to 0.39.9

Update the pinned version across .env.example, both Dockerfiles, and both
Compose stacks so the build arg, image tag, and documented default stay
in sync.

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

Dockerfile.root

@@ -3,7 +3,7 @@
 # ---- build stage: fetch + verify + unzip PocketBase ----
 FROM alpine:3.22 AS build
 
-ARG PB_VERSION=0.39.8
+ARG PB_VERSION=0.39.9
 # provided automatically by buildx (e.g. amd64, arm64)
 ARG TARGETARCH