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.

src/pobsync_backend/apps.py

@@ -0,0 +1,9 @@
+from __future__ import annotations
+
+from django.apps import AppConfig
+
+
+class PobsyncBackendConfig(AppConfig):
+    default_auto_field = "django.db.models.BigAutoField"
+    name = "pobsync_backend"
+    verbose_name = "Pobsync backend"