V79: Newsletters Module — Database Tables

Creates 4 tables for the newsletter broadcast system:

• phoenix_kit_newsletters_lists — Named newsletter lists for segmentation
• phoenix_kit_newsletters_list_members — User membership in lists
• phoenix_kit_newsletters_broadcasts — Email broadcasts (draft/sent/scheduled)
• phoenix_kit_newsletters_deliveries — Per-recipient delivery tracking

All UUIDs use uuid_generate_v7(). All operations are idempotent.