feat(gacha): add /api/gacha wish animation rendered as MP4

A meteor coloured by rarity falls across a night sky, lands in a white
flash, and the label is revealed with its stars popping in. The caller
picks the result and rarity; the route only draws it. Renders as silent
H.264 so Telegram plays it as an animation without GIF banding, and
shares the render slots with /api/gif.

renderer/docs/miti99bot-integration.md

@@ -37,3 +37,11 @@ Send `Authorization: Bearer <API_TOKEN>` with the JSON body.
 
 `X-Wheel-Winner` is URL-encoded. Decode it before displaying if service-side
 winner selection is used.
+
+## Gacha
+
+`/gacha` in miti99bot calls `POST /api/gacha` on the same service and token.
+The bot derives the endpoint from `WHEELOFNAMES_API_URL` by replacing its last
+path segment (`/api/gif` becomes `/api/gacha`), picks the result and rarity
+itself, and sends the MP4 with `sendAnimation`. On any failure it falls back to
+a text reply.