(feature) Add global and effective host config checks

Introduce reusable configuration checks for global settings and effective
host runtime configuration. The checks now surface risky backup settings
such as missing recursive rsync args, missing critical root excludes,
invalid SSH settings, missing credentials, and retention gaps.

Show these checks on the global config form, host edit form, and host
detail page so operators can validate the compounded host/global config
before starting real backup runs.

src/pobsync_backend/views.py

@@ -16,6 +16,7 @@ from pobsync.errors import PobsyncError
 from pobsync.config.defaults import DEFAULT_EXCLUDES, DEFAULT_RSYNC_ARGS
 
 from .backup_runner import queue_backup_run
+from .config_checks import collect_effective_host_config_checks, collect_global_config_checks
 from .forms import (
     CreateHostConfigForm,
     GlobalConfigForm,
@@ -203,6 +204,7 @@ def edit_global_config(request):
             return redirect("dashboard")
     else:
         form = GlobalConfigForm(instance=global_config) if global_config else GlobalConfigForm(initial=_default_global_initial())
+    config_checks = collect_global_config_checks(global_config) if global_config else []
 
     return render(
         request,
@@ -211,6 +213,8 @@ def edit_global_config(request):
             "global_config": global_config,
             "form": form,
             "backup_root": settings.POBSYNC_BACKUP_ROOT,
+            "config_checks": config_checks,
+            "config_check_summary": summarize_self_checks(config_checks),
         },
     )
 
@@ -452,6 +456,8 @@ def apply_host_retention(request, host: str):
 @staff_member_required
 def edit_host_config(request, host: str):
     host_config = get_object_or_404(HostConfig, host=host)
+    global_config = GlobalConfig.objects.filter(name="default").first()
+    config_checks = collect_effective_host_config_checks(host_config, global_config) if global_config else []
     if request.method == "POST":
         form = HostConfigForm(request.POST, instance=host_config)
         if form.is_valid():
@@ -467,6 +473,8 @@ def edit_host_config(request, host: str):
         {
             "host": host_config,
             "form": form,
+            "config_checks": config_checks,
+            "config_check_summary": summarize_self_checks(config_checks),
         },
     )