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/migrations/0001_initial.py

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+from __future__ import annotations
+
+import django.db.models.deletion
+from django.db import migrations, models
+
+
+class Migration(migrations.Migration):
+    initial = True
+
+    dependencies = []
+
+    operations = [
+        migrations.CreateModel(
+            name="GlobalConfig",
+            fields=[
+                ("id", models.BigAutoField(auto_created=True, primary_key=True, serialize=False, verbose_name="ID")),
+                ("created_at", models.DateTimeField(auto_now_add=True)),
+                ("updated_at", models.DateTimeField(auto_now=True)),
+                ("name", models.CharField(default="default", max_length=64, unique=True)),
+                ("backup_root", models.CharField(max_length=512)),
+                ("pobsync_home", models.CharField(default="/opt/pobsync", max_length=512)),
+                ("data", models.JSONField(blank=True, default=dict)),
+            ],
+            options={
+                "verbose_name": "global config",
+                "verbose_name_plural": "global configs",
+            },
+        ),
+        migrations.CreateModel(
+            name="HostConfig",
+            fields=[
+                ("id", models.BigAutoField(auto_created=True, primary_key=True, serialize=False, verbose_name="ID")),
+                ("created_at", models.DateTimeField(auto_now_add=True)),
+                ("updated_at", models.DateTimeField(auto_now=True)),
+                ("host", models.CharField(max_length=255, unique=True)),
+                ("address", models.CharField(max_length=255)),
+                ("enabled", models.BooleanField(default=True)),
+                ("config", models.JSONField(blank=True, default=dict)),
+            ],
+            options={
+                "ordering": ["host"],
+            },
+        ),
+        migrations.CreateModel(
+            name="BackupRun",
+            fields=[
+                ("id", models.BigAutoField(auto_created=True, primary_key=True, serialize=False, verbose_name="ID")),
+                ("run_type", models.CharField(choices=[("scheduled", "Scheduled"), ("manual", "Manual")], default="scheduled", max_length=16)),
+                ("status", models.CharField(choices=[("queued", "Queued"), ("running", "Running"), ("success", "Success"), ("failed", "Failed"), ("cancelled", "Cancelled")], default="queued", max_length=16)),
+                ("started_at", models.DateTimeField(blank=True, null=True)),
+                ("ended_at", models.DateTimeField(blank=True, null=True)),
+                ("snapshot_path", models.CharField(blank=True, max_length=1024)),
+                ("base_path", models.CharField(blank=True, max_length=1024)),
+                ("rsync_exit_code", models.IntegerField(blank=True, null=True)),
+                ("result", models.JSONField(blank=True, default=dict)),
+                ("created_at", models.DateTimeField(auto_now_add=True)),
+                ("host", models.ForeignKey(on_delete=django.db.models.deletion.PROTECT, related_name="runs", to="pobsync_backend.hostconfig")),
+            ],
+            options={
+                "ordering": ["-created_at"],
+            },
+        ),
+        migrations.CreateModel(
+            name="ScheduleConfig",
+            fields=[
+                ("id", models.BigAutoField(auto_created=True, primary_key=True, serialize=False, verbose_name="ID")),
+                ("created_at", models.DateTimeField(auto_now_add=True)),
+                ("updated_at", models.DateTimeField(auto_now=True)),
+                ("cron_expr", models.CharField(max_length=128)),
+                ("user", models.CharField(default="root", max_length=64)),
+                ("enabled", models.BooleanField(default=True)),
+                ("prune", models.BooleanField(default=False)),
+                ("prune_max_delete", models.PositiveIntegerField(default=10)),
+                ("prune_protect_bases", models.BooleanField(default=False)),
+                ("host", models.OneToOneField(on_delete=django.db.models.deletion.CASCADE, related_name="schedule", to="pobsync_backend.hostconfig")),
+            ],
+            options={
+                "ordering": ["host__host"],
+            },
+        ),
+        migrations.CreateModel(
+            name="SnapshotRecord",
+            fields=[
+                ("id", models.BigAutoField(auto_created=True, primary_key=True, serialize=False, verbose_name="ID")),
+                ("kind", models.CharField(choices=[("scheduled", "Scheduled"), ("manual", "Manual"), ("incomplete", "Incomplete")], max_length=16)),
+                ("dirname", models.CharField(max_length=255)),
+                ("path", models.CharField(max_length=1024)),
+                ("status", models.CharField(blank=True, max_length=32)),
+                ("started_at", models.DateTimeField(blank=True, null=True)),
+                ("ended_at", models.DateTimeField(blank=True, null=True)),
+                ("metadata", models.JSONField(blank=True, default=dict)),
+                ("discovered_at", models.DateTimeField(auto_now_add=True)),
+                ("host", models.ForeignKey(on_delete=django.db.models.deletion.CASCADE, related_name="snapshots", to="pobsync_backend.hostconfig")),
+            ],
+            options={
+                "ordering": ["host__host", "-started_at", "dirname"],
+            },
+        ),
+        migrations.AddConstraint(
+            model_name="snapshotrecord",
+            constraint=models.UniqueConstraint(fields=("host", "kind", "dirname"), name="unique_snapshot_per_host_kind"),
+        ),
+    ]

src/pobsync_backend/migrations/__init__.py

@@ -0,0 +1 @@
+