(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/models.py

@@ -14,7 +14,6 @@ class TimestampedModel(models.Model):
 class GlobalConfig(TimestampedModel):
     name = models.CharField(max_length=64, default="default", unique=True)
     backup_root = models.CharField(max_length=512)
-    pobsync_home = models.CharField(max_length=512, default="/opt/pobsync")
     default_ssh_credential = models.ForeignKey(
         "SshCredential",
         on_delete=models.SET_NULL,