refactor(random)!: configure the renderer by base URL as RENDERER_URL

The bot now takes the renderer's base URL and appends each /api/<name>
route itself, instead of taking the full /api/gif endpoint and swapping
its last path segment for /gacha and /genshin. The client, its files, and
its errors are named after the renderer rather than wheelofnames; the
/wheelofnames command keeps its name.

BREAKING CHANGE: WHEELOFNAMES_API_URL is replaced by RENDERER_URL, which
holds the base URL (e.g. http://renderer:3000) rather than the /api/gif
endpoint. compose.yml sets it, so Coolify needs no value.

renderer/docs/miti99bot-integration.md

@@ -1,7 +1,7 @@
 # miti99bot Integration
 
 miti99bot's `/wheelofnames` renders its wheel with this service, configured
-by `WHEELOFNAMES_API_URL` (pointing at `/api/gif`). The service is internal to
+by `RENDERER_URL`, the service's base URL. The service is internal to
 the compose network, so requests carry no credentials. Winner handling stays
 explicit:
 
@@ -43,8 +43,7 @@ winner selection is used.
 ## Gacha
 
 `/gacha` in miti99bot calls `POST /api/gacha` on the same service.
-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
+The bot appends the route to `RENDERER_URL`, picks the result and rarity
 itself, and sends the MP4 with `sendAnimation`. On any failure it falls back to
 a text reply.