overidx: {num} column is INTEGER PRIMARY KEY

overidx: {num} column is INTEGER PRIMARY KEY

INTEGER PRIMARY KEY can be an alias for ROWID in SQLite and is
already unique, so there's no need for a separate UNIQUE(num)
index.

With a smallish ~3K, freshly indexed v2 inbox, this results in a
~40K space savings, reducing over.sqlite3 from 1.375M to 1.335M
(post-VACUUM).

This only affects newly-indexed inboxes; existing DBs will
require manual intervention to take advantage of space savings.

Link: https://www.sqlite.org/rowidtable.html