feat: make Django configs drive backups and scheduling

Treat SQL-backed Django models as the source of truth for pobsync
configuration, exporting runtime YAML only as a compatibility layer for
the existing engine. Add a database-driven scheduler command, Docker
scheduler services, schedule run-state fields, and tests for scheduler,
config export, and retention behavior.

src/pobsync_backend/migrations/0002_schedule_run_state.py

@@ -0,0 +1,32 @@
+from __future__ import annotations
+
+from django.db import migrations, models
+
+
+class Migration(migrations.Migration):
+    dependencies = [
+        ("pobsync_backend", "0001_initial"),
+    ]
+
+    operations = [
+        migrations.AddField(
+            model_name="scheduleconfig",
+            name="last_due_key",
+            field=models.CharField(blank=True, max_length=32),
+        ),
+        migrations.AddField(
+            model_name="scheduleconfig",
+            name="last_finished_at",
+            field=models.DateTimeField(blank=True, null=True),
+        ),
+        migrations.AddField(
+            model_name="scheduleconfig",
+            name="last_started_at",
+            field=models.DateTimeField(blank=True, null=True),
+        ),
+        migrations.AddField(
+            model_name="scheduleconfig",
+            name="last_status",
+            field=models.CharField(blank=True, max_length=16),
+        ),
+    ]