feat: add Django backend foundation and Docker runtime

Add a Django admin-backed management layer for pobsync configs, runs,
snapshots, and schedules. Keep the existing CLI engine as the execution
source of truth, add import/run management commands, and provide SQLite
default plus optional MariaDB Docker Compose support.

scripts/docker-entrypoint

@@ -0,0 +1,8 @@
+#!/bin/sh
+set -eu
+
+mkdir -p "$(dirname "${POBSYNC_SQLITE_PATH:-/var/lib/pobsync/pobsync.sqlite3}")"
+
+python manage.py migrate --noinput
+
+exec "$@"