Fix various minor inconsistencies between the production database schema
and the declared schema in db/structure.sql.
* tags.category was a smallint instead of an integer in production.
* The unique_schema_migrations index didn't exist outside production.
* The index_posts_on_tag_index index was called index_posts_on_tags_index
outside production.
* The posts.tag_index column didn't have a statistics target defined
outside production.
* ID sequences didn't have `AS integer` defined in production.
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