feat(misc): add /gacha Genshin-style wish picker

Options take an optional *4 or *5 rarity tag; untagged options are 3 stars.
A pick rolls a tier at Genshin base rates over the tiers present, then an
option uniformly within it, so untagged input matches /random. The wish
animation renders as MP4 on the wheelofnames service at /api/gacha, with a
text fallback when the renderer is unset or fails.

cmd/server/command_menu_test.go

@@ -67,6 +67,7 @@ func TestCommandDiscovery_AllPublicCommandsHaveSafeMetadata(t *testing.T) {
 		"gold_topup":           "<vnd_amount>",
 		"gold_buy":             "<luong>",
 		"gold_sell":            "<luong>",
+		"gacha":                "<option,...>",
 		"lol":                  "[date]",
 		"loldle":               "[champion]",
 		"monkeyd_crawl":        "<url> [font_size]",