Add seed_translations management command for DeepL

[Raghaddahi]

Description

Adds a Django management command to bulk-seed initial page translations into target locales using DeepL (via wagtail-localize's machine translator).

Useful for the first-time case where manually translating every page via the admin is tedious. The command automates the same steps the admin would do — create the target locale, copy the source page, run machine translation — and is idempotent for reruns.

Usage

python manage.py seed_translations --locale fr
python manage.py seed_translations --locale fr --page manage-snippets
python manage.py seed_translations --locale fr --subtree how-to

Notes

• Pages are always saved as drafts (human review before publishing).
• Target locales must be in WAGTAIL_GUIDE_LANGUAGES; the source locale (en) is rejected.
• Translator comes from WAGTAILLOCALIZE_MACHINE_TRANSLATOR (provider-agnostic); currently configured for DeepL free tier.
• DeepL-unsupported locales are skipped gracefully.
• Idempotent: reruns skip existing pairs and don't double-bill DeepL (segments already translated are skipped).
• Abort on translator failure.

AI usage

Code was written with AI assistance (opencode); the user reviewed and tested each step inline before committing. PR description drafted by the AI agent and reviewed by the user.