Deploy: create the data directories instead of assuming Docker will

"Bind mount failed: '/volume1/docker/webmail/data/postgres' doesn't exists" on
Synology after the data directory was removed. Docker on ordinary Linux creates
a missing bind-mount source silently; DSM's daemon refuses and fails the
container. Every service mounted its own subdirectory, so every one of them
needed to already exist — including the `init` service whose whole job was to
create them, which could therefore never run.

- init now mounts DATA_ROOT itself, once, and creates postgres/blobs/backups/
  redis/meili/state inside it. Only DATA_ROOT has to pre-exist.
- postgres gained the init dependency it was missing, so its data directory is
  never demanded before init has made it.
- init only runs `chown -R` when ownership is actually wrong, rather than
  walking the whole blob store on every start.
- webmail.sh up creates DATA_ROOT and, if it cannot, says so with the mkdir to
  run rather than leaving the daemon to produce the error above.

Verified from an empty DATA_ROOT: init creates all six directories, postgres
starts after it, migrations apply, stack serves.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01Nux3LCu4zP9nih4BRhPyHw

deploy/webmail.sh

@@ -326,10 +326,15 @@ cmd_up() {
         ylw "SEARCH_BACKEND=meili — starting the Meilisearch container too"
         profile=(--profile meili)
     }
-    # Best-effort only: Docker creates any missing bind-mount path itself, and
-    # the `init` service fixes ownership. This line must not be fatal, because
-    # DATA_ROOT is typically root-owned while you may be running as yourself.
-    mkdir -p "${DATA_ROOT}"/{postgres,blobs,redis,backups,meili,state} 2>/dev/null || true
+    # DATA_ROOT must exist before compose runs: Synology's Docker daemon will
+    # not create a missing bind-mount source, it just fails the container. The
+    # `init` service creates the subdirectories from there.
+    mkdir -p "${DATA_ROOT}" 2>/dev/null || true
+    if [ ! -d "${DATA_ROOT}" ]; then
+        die "cannot create ${DATA_ROOT} (running as $(id -un)). Create it first:
+    sudo mkdir -p ${DATA_ROOT}
+then re-run this command."
+    fi
     detect_compose
     # shellcheck disable=SC2086
     compose "${profile[@]}" up -d "${build[@]}" $COMPOSE_WAIT