Three-stage build: bun compiles the frontend, cargo embeds it into the
release binary, and the runtime stage carries only that binary on
debian:trixie-slim as an unprivileged user.
compose.yml pins DIARY_BIND and DIARY_DATA_DIR over whatever .env says,
keeps state in a named volume, publishes on localhost only and probes the
port with bash so the image needs no curl. compose.override.yml.example
shows the traefik overlay for the personal host.
