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.

manage.py

@@ -0,0 +1,16 @@
+#!/usr/bin/env python3
+from __future__ import annotations
+
+import os
+import sys
+
+
+def main() -> None:
+    os.environ.setdefault("DJANGO_SETTINGS_MODULE", "pobsync_server.settings")
+    from django.core.management import execute_from_command_line
+
+    execute_from_command_line(sys.argv)
+
+
+if __name__ == "__main__":
+    main()