Partial index (filter: "Status" = 0) replaces the unconditional unique
index on (GuildId, InviteeViewerId). Old Rejected/Canceled rows no
longer conflict when a new Pending invite is inserted for the same pair.
Adds regression tests: Reinvite_after_reject_succeeds and
Reinvite_after_cancel_succeeds.
Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>