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/management/commands/import_pobsync_configs.py

@@ -0,0 +1,48 @@
+from __future__ import annotations
+
+from pathlib import Path
+from typing import Any
+
+from django.conf import settings
+from django.core.management.base import BaseCommand, CommandError
+
+from pobsync.config.load import load_global_config, load_host_config
+from pobsync.paths import PobsyncPaths
+from pobsync_backend.models import GlobalConfig, HostConfig
+
+
+class Command(BaseCommand):
+    help = "Import pobsync YAML configs into the Django database."
+
+    def add_arguments(self, parser) -> None:
+        parser.add_argument("--prefix", default=settings.POBSYNC_HOME, help="Pobsync home directory")
+
+    def handle(self, *args: Any, **options: Any) -> None:
+        paths = PobsyncPaths(home=Path(options["prefix"]))
+        if not paths.global_config_path.exists():
+            raise CommandError(f"Missing global config: {paths.global_config_path}")
+
+        global_cfg = load_global_config(paths.global_config_path)
+        GlobalConfig.objects.update_or_create(
+            name="default",
+            defaults={
+                "backup_root": global_cfg["backup_root"],
+                "pobsync_home": global_cfg.get("pobsync_home", str(paths.home)),
+                "data": global_cfg,
+            },
+        )
+
+        count = 0
+        for host_path in sorted(paths.hosts_dir.glob("*.yaml")):
+            host_cfg = load_host_config(host_path)
+            HostConfig.objects.update_or_create(
+                host=host_cfg["host"],
+                defaults={
+                    "address": host_cfg["address"],
+                    "config": host_cfg,
+                    "enabled": True,
+                },
+            )
+            count += 1
+
+        self.stdout.write(self.style.SUCCESS(f"Imported global config and {count} host config(s)."))