V66: Make legacy user_id columns nullable on posts module tables.

After the UUID cleanup, posts schemas only set user_uuid — they never set the legacy user_id bigint column. Five tables still have user_id bigint NOT NULL, causing inserts to fail with not_null_violation.

This migration makes user_id nullable on all affected tables.

Tables Fixed

• phoenix_kit_post_groups (V29)
• phoenix_kit_post_comments (V29)
• phoenix_kit_post_likes (V29)
• phoenix_kit_post_dislikes (V48)
• phoenix_kit_post_mentions (V29)