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/management/commands/configure_pobsync_host.py

@@ -0,0 +1,65 @@
+from __future__ import annotations
+
+from typing import Any
+
+from django.core.management.base import BaseCommand, CommandError
+
+from pobsync.cli import parse_retention
+from pobsync.util import sanitize_host
+from pobsync_backend.models import GlobalConfig, HostConfig
+
+
+class Command(BaseCommand):
+    help = "Create or update a SQL-backed host pobsync configuration."
+
+    def add_arguments(self, parser) -> None:
+        parser.add_argument("host")
+        parser.add_argument("--address", required=True)
+        parser.add_argument("--ssh-user", default="")
+        parser.add_argument("--ssh-port", type=int, default=None)
+        parser.add_argument("--source-root", default="")
+        parser.add_argument("--include", action="append", default=[])
+        parser.add_argument("--exclude-add", action="append", default=[])
+        parser.add_argument("--exclude-replace", action="append", default=None)
+        parser.add_argument("--rsync-extra-arg", action="append", default=[])
+        parser.add_argument("--retention", default=None)
+        parser.add_argument("--disabled", action="store_true")
+        parser.add_argument("--force", action="store_true", help="Update existing host")
+
+    def handle(self, *args: Any, **options: Any) -> None:
+        host = sanitize_host(options["host"])
+        if HostConfig.objects.filter(host=host).exists() and not options["force"]:
+            raise CommandError(f"HostConfig {host!r} already exists; use --force to update")
+
+        retention = self._retention(options["retention"])
+        defaults = {
+            "address": options["address"],
+            "enabled": not options["disabled"],
+            "ssh_user": options["ssh_user"],
+            "ssh_port": options["ssh_port"],
+            "source_root": options["source_root"],
+            "includes": list(options["include"]),
+            "excludes_add": [] if options["exclude_replace"] is not None else list(options["exclude_add"]),
+            "excludes_replace": options["exclude_replace"],
+            "rsync_extra_args": list(options["rsync_extra_arg"]),
+            "retention_daily": retention["daily"],
+            "retention_weekly": retention["weekly"],
+            "retention_monthly": retention["monthly"],
+            "retention_yearly": retention["yearly"],
+        }
+        _obj, created = HostConfig.objects.update_or_create(host=host, defaults=defaults)
+        action = "Created" if created else "Updated"
+        self.stdout.write(self.style.SUCCESS(f"{action} HostConfig {host!r}."))
+
+    def _retention(self, value: str | None) -> dict[str, int]:
+        if value:
+            return parse_retention(value)
+        global_config = GlobalConfig.objects.filter(name="default").first()
+        if global_config is None:
+            return {"daily": 14, "weekly": 8, "monthly": 12, "yearly": 0}
+        return {
+            "daily": global_config.retention_daily,
+            "weekly": global_config.retention_weekly,
+            "monthly": global_config.retention_monthly,
+            "yearly": global_config.retention_yearly,
+        }