Editable AI prompt templates in the UI (with restore-to-default)

Description Is your feature request related to a problem? Since the Jinja2 refactor (#13698), the AI prompts live in clean template files (paperless_ai/prompts/*.j2) — but there's no supported way to customize them (render.py uses a PackageLoader only). This came up before in #12871, and the use cases are still real: • Language control: emit tags/titles directly in the target language instead of relying on the localization pass • Title conventions: enforce a house style, e.g. <doctype> <invoice number> - <correspondent> • Domain terminology: steer the model toward industry-specific vocabulary Describe the solution you'd like Editable prompt templates in the existing AI settings section of the UI, stored in the database alongside the other AI configuration (as suggested in #12871): • A text area per prompt (classification, localization, …), pre-filled with the shipped default • An empty value means "use t…