refactor: promote backup configuration to structured SQL fields

Add explicit Django model fields for global and host backup settings,
including SSH, rsync, source, excludes, and retention configuration.
Populate them from legacy JSON during migration, make the config
repository prefer structured fields, and update import/admin/tests around
the SQL-first configuration model.

src/pobsync_backend/admin.py

@@ -7,8 +7,28 @@ from .models import BackupRun, GlobalConfig, HostConfig, ScheduleConfig, Snapsho
 
 @admin.register(GlobalConfig)
 class GlobalConfigAdmin(admin.ModelAdmin):
-    list_display = ("name", "backup_root", "updated_at")
+    list_display = ("name", "backup_root", "ssh_user", "ssh_port", "updated_at")
     readonly_fields = ("created_at", "updated_at")
+    fieldsets = (
+        (None, {"fields": ("name", "backup_root", "pobsync_home")}),
+        ("SSH", {"fields": ("ssh_user", "ssh_port", "ssh_options")}),
+        (
+            "Rsync",
+            {
+                "fields": (
+                    "rsync_binary",
+                    "rsync_args",
+                    "rsync_extra_args",
+                    "rsync_timeout_seconds",
+                    "rsync_bwlimit_kbps",
+                )
+            },
+        ),
+        ("Defaults", {"fields": ("default_source_root", "default_destination_subdir", "excludes_default")}),
+        ("Retention defaults", {"fields": ("retention_daily", "retention_weekly", "retention_monthly", "retention_yearly")}),
+        ("Legacy JSON", {"fields": ("data",), "classes": ("collapse",)}),
+        ("Timestamps", {"fields": ("created_at", "updated_at"), "classes": ("collapse",)}),
+    )
 
 
 @admin.register(HostConfig)
@@ -17,6 +37,15 @@ class HostConfigAdmin(admin.ModelAdmin):
     list_filter = ("enabled",)
     search_fields = ("host", "address")
     readonly_fields = ("created_at", "updated_at")
+    fieldsets = (
+        (None, {"fields": ("host", "address", "enabled")}),
+        ("SSH override", {"fields": ("ssh_user", "ssh_port")}),
+        ("Source", {"fields": ("source_root", "includes", "excludes_add", "excludes_replace")}),
+        ("Rsync override", {"fields": ("rsync_extra_args",)}),
+        ("Retention", {"fields": ("retention_daily", "retention_weekly", "retention_monthly", "retention_yearly")}),
+        ("Legacy JSON", {"fields": ("config",), "classes": ("collapse",)}),
+        ("Timestamps", {"fields": ("created_at", "updated_at"), "classes": ("collapse",)}),
+    )
 
 
 @admin.register(BackupRun)