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/config_repository.py

@@ -0,0 +1,64 @@
+from __future__ import annotations
+
+from pathlib import Path
+from typing import Any
+
+from django.core.exceptions import ObjectDoesNotExist
+
+from pobsync.config.schemas import GLOBAL_SCHEMA, HOST_SCHEMA
+from pobsync.paths import PobsyncPaths
+from pobsync.util import write_yaml_atomic
+from pobsync.validate import validate_dict
+
+from .models import GlobalConfig, HostConfig
+
+
+class ConfigRepositoryError(RuntimeError):
+    pass
+
+
+def _global_yaml_data(global_config: GlobalConfig) -> dict[str, Any]:
+    data = dict(global_config.data or {})
+    data["backup_root"] = global_config.backup_root
+    data["pobsync_home"] = global_config.pobsync_home
+    return validate_dict(data, GLOBAL_SCHEMA, path="global")
+
+
+def _host_yaml_data(host_config: HostConfig) -> dict[str, Any]:
+    data = dict(host_config.config or {})
+    data["host"] = host_config.host
+    data["address"] = host_config.address
+    return validate_dict(data, HOST_SCHEMA, path="host")
+
+
+def export_global_config(prefix: Path, name: str = "default") -> Path:
+    try:
+        global_config = GlobalConfig.objects.get(name=name)
+    except ObjectDoesNotExist as exc:
+        raise ConfigRepositoryError(f"Missing GlobalConfig {name!r}") from exc
+
+    paths = PobsyncPaths(home=prefix)
+    write_yaml_atomic(paths.global_config_path, _global_yaml_data(global_config))
+    return paths.global_config_path
+
+
+def export_host_config(prefix: Path, host: str) -> Path:
+    try:
+        host_config = HostConfig.objects.get(host=host, enabled=True)
+    except ObjectDoesNotExist as exc:
+        raise ConfigRepositoryError(f"Missing enabled HostConfig {host!r}") from exc
+
+    paths = PobsyncPaths(home=prefix)
+    target = paths.hosts_dir / f"{host_config.host}.yaml"
+    write_yaml_atomic(target, _host_yaml_data(host_config))
+    return target
+
+
+def export_runtime_configs(prefix: Path, host: str | None = None) -> list[Path]:
+    written = [export_global_config(prefix)]
+    hosts = HostConfig.objects.filter(enabled=True).order_by("host")
+    if host is not None:
+        hosts = hosts.filter(host=host)
+    for host_config in hosts:
+        written.append(export_host_config(prefix, host_config.host))
+    return written