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.

docs/deploy-coolify-selfhosted.md

@@ -85,6 +85,13 @@ render or upload failure edits that same message into the plain text winner
 instead. With no renderer configured there is no holding message — the winner
 reply is immediate.
 
+`/gacha` uses the same service and token: the bot swaps the URL's last path
+segment, so `.../api/gif` becomes `.../api/gacha`. It renders a 7-second
+`640x360` silent MP4 wish animation, posts `Wishing...` while it renders, and
+falls back to a text reply such as `★★★★★ Pizza` on the same failures. Options
+tagged `*4` or `*5` are rarer, following Genshin base rates (5★ 0.6%, 4★ 5.1%,
+3★ 94.3%) over the tiers present; untagged options are 3★.
+
 ## 1. MongoDB Atlas (M0)
 
 1. Create a free **M0** cluster (512 MB — ample for the tiny paper-trading KV).