refactor(status-page): apply review feedback on theme-fields backend

- StatusPage Mongoose model: import DEFAULT_STATUS_PAGE_THEME / DEFAULT_STATUS_PAGE_THEME_MODE from types/statusPage so the four sources of the default literal stay locked together
- openapi.json: add explicit "default" alongside the enum on StatusPageTheme and StatusPageThemeMode component schemas; documentation already mentioned the defaults but the JSON didn't carry them
- statusPageController: add JSDoc-style comments above withoutThemeFields and applyDefaultTheme spelling out the asymmetric strip-on-write / force-on-read semantics so the next reviewer doesn't have to re-derive the design

server/openapi.json

@@ -3354,11 +3354,13 @@
 			"StatusPageTheme": {
 				"type": "string",
 				"enum": ["refined", "modern", "bold", "editorial"],
+				"default": "refined",
 				"description": "Visual theme for the public status page. Defaults to 'refined'."
 			},
 			"StatusPageThemeMode": {
 				"type": "string",
 				"enum": ["auto", "light", "dark"],
+				"default": "auto",
 				"description": "Forces a light/dark mode on the public page. 'auto' follows the visitor's OS preference. Defaults to 'auto'."
 			},
 			"SuccessResponse": {