(bugfix) Guard dry-run logs and recursive rsync defaults

Clear the reused dry-run rsync log before each dry-run so run details
only show output from the current execution.

Populate new Django global configs with the existing safe rsync and
exclude defaults, including archive mode and standard pseudo-filesystem
exclusions.

Add a host check that fails when effective rsync args do not include
archive or recursive transfer, preventing real backups that only report
"skipping directory .".

src/pobsync_backend/views.py

@@ -13,6 +13,7 @@ from django.shortcuts import get_object_or_404, redirect, render
 from django.views.decorators.http import require_POST
 
 from pobsync.errors import PobsyncError
+from pobsync.config.defaults import DEFAULT_EXCLUDES, DEFAULT_RSYNC_ARGS
 
 from .backup_runner import queue_backup_run
 from .forms import (
@@ -519,7 +520,9 @@ def _default_global_initial() -> dict[str, object]:
         "ssh_user": "root",
         "ssh_port": 22,
         "rsync_binary": "rsync",
+        "rsync_args": DEFAULT_RSYNC_ARGS,
         "default_source_root": "/",
+        "excludes_default": DEFAULT_EXCLUDES,
         "retention_daily": 14,
         "retention_weekly": 8,
         "retention_monthly": 12,