refactor: stop using legacy JSON for runtime config

Build runtime pobsync configuration exclusively from structured SQL
fields, leaving legacy JSON only for import and audit context. Add
SQL-first management commands for global and host configuration and
cover them with tests.

src/pobsync_backend/tests/test_configure_commands.py

@@ -0,0 +1,56 @@
+from __future__ import annotations
+
+from io import StringIO
+
+from django.core.management import call_command
+from django.test import TestCase
+
+from pobsync_backend.config_source import DjangoConfigSource
+from pobsync_backend.models import GlobalConfig, HostConfig
+
+
+class ConfigureCommandsTests(TestCase):
+    def test_configure_global_creates_structured_config(self) -> None:
+        out = StringIO()
+
+        call_command(
+            "configure_pobsync_global",
+            backup_root="/backups",
+            pobsync_home="/opt/pobsync",
+            retention="daily=3,weekly=2,monthly=1,yearly=0",
+            stdout=out,
+        )
+
+        config = GlobalConfig.objects.get(name="default")
+        self.assertEqual(config.backup_root, "/backups")
+        self.assertEqual(config.retention_daily, 3)
+        self.assertIn("Created GlobalConfig", out.getvalue())
+
+    def test_configure_host_uses_global_retention_defaults(self) -> None:
+        GlobalConfig.objects.create(
+            name="default",
+            backup_root="/backups",
+            retention_daily=5,
+            retention_weekly=4,
+            retention_monthly=3,
+            retention_yearly=2,
+        )
+        out = StringIO()
+
+        call_command(
+            "configure_pobsync_host",
+            "web-01",
+            address="web-01.example.test",
+            exclude_add=["/tmp/***"],
+            rsync_extra_arg=["--delete"],
+            stdout=out,
+        )
+
+        host = HostConfig.objects.get(host="web-01")
+        self.assertEqual(host.retention_daily, 5)
+        self.assertEqual(host.excludes_add, ["/tmp/***"])
+        self.assertEqual(host.rsync_extra_args, ["--delete"])
+
+        effective = DjangoConfigSource().effective_config_for_host("web-01")
+        self.assertEqual(effective["retention"]["yearly"], 2)
+        self.assertEqual(effective["excludes_effective"], ["/tmp/***"])