Add migration to cleanup dead invites

Fixes #350.

state/migrations/20231019153023_cleanup_dead_invites.sql

@@ -0,0 +1,22 @@
+-- +goose Up
+-- +goose StatementBegin
+WITH
+    dead_invites(user_id, room_id) AS (
+        SELECT syncv3_invites.user_id, syncv3_invites.room_id
+        FROM syncv3_invites
+            JOIN syncv3_rooms USING (room_id)
+            JOIN syncv3_snapshots ON (syncv3_snapshots.snapshot_id = syncv3_rooms.current_snapshot_id)
+            JOIN syncv3_events ON (
+                    event_nid = ANY (membership_events)
+                AND state_key = syncv3_invites.user_id
+                AND NOT (membership = 'invite' OR membership = '_invite')
+            )
+    )
+DELETE FROM syncv3_invites
+WHERE (user_id, room_id) IN (SELECT * FROM dead_invites);
+-- +goose StatementEnd
+
+-- +goose Down
+-- +goose StatementBegin
+-- no-op
+-- +goose StatementEnd