fix: create data subdirectories on first container start

On a host that has never run the service, the mounted data volume is
empty. Ownership was being corrected for a subdirectory that did not
exist yet, which aborted the entrypoint before the application started
— exactly the situation a fresh deployment creates, and the reason the
container came up on the existing Pi but would not have elsewhere.

Creating the directory has to happen at runtime rather than during the
build, because the volume mount hides whatever the image places there.

entrypoint.sh

@@ -4,7 +4,9 @@ set -e
 APP_UID=${APP_UID:-1000}
 APP_GID=${APP_GID:-1000}
 
-# Fix ownership of mounted volumes so the app user can write to them
+# Mounted volumes start out empty on a fresh host, and a mount hides anything the
+# image created at these paths — so create the directories here, not at build time
+mkdir -p /app/tmp /app/data/images
 chown "${APP_UID}:${APP_GID}" /app/tmp /app/data /app/data/images
 
 # Self-update yt-dlp on every container start (Instagram/TikTok break old versions).