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_config_repository.py

@@ -30,6 +30,7 @@ class ConfigRepositoryTests(TestCase):
                     "backup_root": "/ignored",
                     "pobsync_home": "/ignored",
                     "ssh": {"user": "ignored", "port": 22, "options": []},
+                    "unknown": "must-not-leak",
                     "retention_defaults": {"daily": 99, "weekly": 99, "monthly": 99, "yearly": 99},
                 },
             )
@@ -48,6 +49,7 @@ class ConfigRepositoryTests(TestCase):
                     "address": "ignored",
                     "retention": {"daily": 99, "weekly": 99, "monthly": 99, "yearly": 99},
                     "excludes_add": ["/ignored/***"],
+                    "unknown": "must-not-leak",
                 },
             )
 
@@ -65,3 +67,5 @@ class ConfigRepositoryTests(TestCase):
             self.assertEqual(host_cfg["address"], "web-01.example.test")
             self.assertEqual(host_cfg["retention"]["daily"], 7)
             self.assertEqual(host_cfg["excludes_add"], ["/tmp/***"])
+            self.assertNotIn("unknown", global_cfg)
+            self.assertNotIn("unknown", host_cfg)