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/tests/test_scheduler.py

@@ -0,0 +1,60 @@
+from __future__ import annotations
+
+from datetime import datetime
+from pathlib import Path
+from unittest.mock import patch
+from zoneinfo import ZoneInfo
+
+from django.test import SimpleTestCase, TestCase
+
+from pobsync_backend.management.commands.run_pobsync_scheduler import Command
+from pobsync_backend.models import HostConfig, ScheduleConfig
+from pobsync_backend.scheduler import due_key, is_due
+
+
+class SchedulerTests(SimpleTestCase):
+    def test_daily_time_is_due_only_on_matching_minute(self) -> None:
+        moment = datetime(2026, 5, 19, 2, 15, tzinfo=ZoneInfo("UTC"))
+
+        self.assertTrue(is_due("15 2 * * *", moment))
+        self.assertFalse(is_due("16 2 * * *", moment))
+
+    def test_step_values_are_supported(self) -> None:
+        moment = datetime(2026, 5, 19, 2, 30, tzinfo=ZoneInfo("UTC"))
+
+        self.assertTrue(is_due("*/15 * * * *", moment))
+        self.assertFalse(is_due("*/20 * * * *", moment))
+
+    def test_sunday_allows_zero_and_seven(self) -> None:
+        sunday = datetime(2026, 5, 24, 2, 0, tzinfo=ZoneInfo("UTC"))
+
+        self.assertTrue(is_due("0 2 * * 0", sunday))
+        self.assertTrue(is_due("0 2 * * 7", sunday))
+
+    def test_due_key_has_minute_granularity(self) -> None:
+        moment = datetime(2026, 5, 19, 2, 15, 45, tzinfo=ZoneInfo("UTC"))
+
+        self.assertEqual(due_key(moment), "202605190215")
+
+
+class SchedulerCommandTests(TestCase):
+    def test_run_due_executes_schedule_once_per_minute(self) -> None:
+        host = HostConfig.objects.create(
+            host="web-01",
+            address="web-01.example.test",
+            config={
+                "retention": {"daily": 7, "weekly": 4, "monthly": 3, "yearly": 1},
+            },
+        )
+        ScheduleConfig.objects.create(host=host, cron_expr="* * * * *")
+
+        command = Command()
+        with patch("pobsync_backend.management.commands.run_pobsync_scheduler.call_command") as call:
+            first_count = command._run_due(prefix=Path("/opt/pobsync"), dry_run=True)
+            second_count = command._run_due(prefix=Path("/opt/pobsync"), dry_run=True)
+
+        self.assertEqual(first_count, 1)
+        self.assertEqual(second_count, 0)
+        self.assertEqual(call.call_count, 1)
+        schedule = ScheduleConfig.objects.get(host=host)
+        self.assertEqual(schedule.last_status, "success")