AI-assisted translation pipeline + community contributor flow

Two layered flows on top of the i18n foundation. (1) Maintainer flow: when new English keys are added to messages/en.json, a CI step (or admin-triggered "Suggest translations" button) runs them through the BYOK AI provider with strict guardrails - preserves ICU placeholders ({count}, {name}), preserves Markdown, preserves brand terms (Hollahoop, post, board, changelog). Output lands in a translation_suggestions table and surfaces in an admin review queue at /app/{org}/translations - never ships live until a human approves. (2) Community flow: signed-in users can submit translation improvements via an in-app "Suggest a better translation" affordance; admins approve/reject from the same queue.

Bonus: optional "Translate this post" button on user-generated content (posts, comments, changelog entries) - same AI provider, same diff-then-accept UX as AI Suggest. Off by default per-project. Depends on the AI provider settings card. Falls back to displaying the original locale when no key is configured.