Enhancement: support using remote OCR engines selectively (#13633)

* Backend changes and migration for remote OCR Config

* Backend tests

* Frontend stuff, with sections

* Docs

* Update test_tesseract_parser.py

* Actually we cant use this any more, in case settings are in app config

* Dont mark entire test file for db, use a mock for empty engine settings

docs/usage.md

@@ -1090,7 +1090,8 @@ Paperless-ngx supports performing OCR on documents using remote services. At the
 [Microsoft's Azure "Document Intelligence" service](https://azure.microsoft.com/en-us/products/ai-services/ai-document-intelligence).
 This is of course a paid service (with a free tier) which requires an Azure account and subscription. Azure AI is not affiliated with
 Paperless-ngx in any way. When enabled, Paperless-ngx will automatically send appropriate documents to Azure for OCR processing, bypassing
-the local OCR engine. See the [configuration](configuration.md#PAPERLESS_REMOTE_OCR_ENGINE) options for more details.
+the local OCR engine. See the [configuration](configuration.md#PAPERLESS_REMOTE_OCR_ENGINE) options for more details. These
+settings can be supplied as environment variables or via **Application Configuration**.
 
 Additionally, when using a commercial service with this feature, consider both potential costs as well as any associated file size
 or page limitations (e.g. with a free tier).