bug: import rush matches from valve

[lukepolo]

• Rush imports were always rejected. Rush's seed pool has one map, so tai_match materializes rush_001 as soon as the import inserts the match. The importer's own map then trips check_match_map_count ("maximum number of picked maps"), and the import rolls back and ends up Failed. insertMatchMap now clears the trigger-made map first.
• Rush detection is game_mode 6 or a rush_ map. Rush has no skill group, so rank_type can't identify it, and cs_rush is an unrelated hostage map.
• Presence: rush counts as a matchmaking mode, so finishing a Rush match triggers the history poll.
• persist_imported_demo skips Valve rank history and the premier_rank snapshot for Rush, because its scoreboard rank belongs to another ladder.
• No metadata apply is needed; the SQL function is re-applied on boot. Rush imports that already failed can be retried from the external matches page.
• Related: bug: rank type from the rank update, not the scoreboard demo-parser#9 (rank type from the rank update), bug: rush filter for imported valve matches web#609 (Rush filter).