V88: Publishing schema V2 — restructure posts/versions/contents.

Posts become a minimal routing shell. Versions become the source of truth for published state and metadata. Contents hold per-language title + body.

Changes:

• Add active_version_uuid to posts (FK → versions, nullable)
• Add trashed_at to posts (replaces status-based soft delete)
• Add published_at to versions (moved from posts)
• Add title_i18n and description_i18n JSONB to groups (for future i18n)
• Data migration: populate new columns from existing data
• Drop legacy post columns: scheduled_at, status, published_at, primary_language, data
• Drop obsolete indexes (scheduled, group_status, group_published_at)
• Add new indexes for active_version_uuid, trashed_at, and version published_at