V68: Allow NULL slug for timestamp-mode publishing posts

Timestamp-mode posts are identified by (post_date, post_time), not by slug. The NOT NULL constraint on slug was incorrectly applied to all modes.

Changes

1. Drop NOT NULL constraint on slug column in phoenix_kit_publishing_posts
2. Replace unique index idx_publishing_posts_group_slug with a partial index that only enforces uniqueness for slug-mode posts (where slug IS NOT NULL)
3. Add unique index on (group_uuid, post_date, post_time) for timestamp-mode posts

All operations are idempotent.