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/admin.py

@@ -0,0 +1,40 @@
+from __future__ import annotations
+
+from django.contrib import admin
+
+from .models import BackupRun, GlobalConfig, HostConfig, ScheduleConfig, SnapshotRecord
+
+
+@admin.register(GlobalConfig)
+class GlobalConfigAdmin(admin.ModelAdmin):
+    list_display = ("name", "backup_root", "updated_at")
+    readonly_fields = ("created_at", "updated_at")
+
+
+@admin.register(HostConfig)
+class HostConfigAdmin(admin.ModelAdmin):
+    list_display = ("host", "address", "enabled", "updated_at")
+    list_filter = ("enabled",)
+    search_fields = ("host", "address")
+    readonly_fields = ("created_at", "updated_at")
+
+
+@admin.register(BackupRun)
+class BackupRunAdmin(admin.ModelAdmin):
+    list_display = ("host", "run_type", "status", "started_at", "ended_at", "snapshot_path")
+    list_filter = ("run_type", "status", "started_at")
+    search_fields = ("host__host", "snapshot_path")
+
+
+@admin.register(SnapshotRecord)
+class SnapshotRecordAdmin(admin.ModelAdmin):
+    list_display = ("host", "kind", "dirname", "status", "started_at")
+    list_filter = ("kind", "status", "started_at")
+    search_fields = ("host__host", "dirname", "path")
+
+
+@admin.register(ScheduleConfig)
+class ScheduleConfigAdmin(admin.ModelAdmin):
+    list_display = ("host", "cron_expr", "enabled", "prune", "updated_at")
+    list_filter = ("enabled", "prune")
+    search_fields = ("host__host", "cron_expr")