(refactor) Remove pobsync_home from global config

Drop the obsolete pobsync_home field from GlobalConfig and remove it from
runtime config generation, form saves, and configuration commands.

The runtime state root now comes exclusively from POBSYNC_HOME/settings,
which keeps the Django model focused on backup behavior instead of install
layout.

src/pobsync_backend/management/commands/configure_pobsync_global.py

@@ -1,9 +1,7 @@
 from __future__ import annotations
 
-from pathlib import Path
 from typing import Any
 
-from django.conf import settings
 from django.core.management.base import BaseCommand, CommandError
 
 from pobsync.config.retention import parse_retention
@@ -18,7 +16,6 @@ class Command(BaseCommand):
     def add_arguments(self, parser) -> None:
         parser.add_argument("--name", default="default")
         parser.add_argument("--backup-root", required=True)
-        parser.add_argument("--pobsync-home", default=settings.POBSYNC_HOME, help="Runtime state root")
         parser.add_argument("--ssh-user", default="root")
         parser.add_argument("--ssh-port", type=int, default=22)
         parser.add_argument("--source-root", default="/")
@@ -30,11 +27,9 @@ class Command(BaseCommand):
         if not is_absolute_non_root(backup_root):
             raise CommandError("--backup-root must be an absolute path and must not be '/'")
 
-        pobsync_home = str(Path(options["pobsync_home"]))
         retention = parse_retention(options["retention"])
         defaults = {
             "backup_root": backup_root,
-            "pobsync_home": pobsync_home,
             "ssh_user": options["ssh_user"],
             "ssh_port": options["ssh_port"],
             "ssh_options": ["-oBatchMode=yes", "-oStrictHostKeyChecking=accept-new"],