docs: document Opper as an OpenAI-compatible service (#2769)

Add an Opper entry to the OpenAI section of the README, after the
existing service entries. Opper is an EU-hosted AI gateway with an
OpenAI-compatible API at https://api.opper.ai/v3/compat. One key gives
access to 700+ models from 50+ providers.

Docs-only change.

Disclosure: Opper is my startup.

Co-authored-by: Claude Opus 5.5 (1M context) <noreply@anthropic.com>
Co-authored-by: leo <longshuang@msn.cn>

Author: 3 people

README.md

@@ -206,6 +206,12 @@ For `Cheaper Inference`:
 * `API Key` should be a Cheaper Inference API key (prefixed with `ci_live_`)
 * A good starting `Model` is `gpt-5.4-mini`
 
+For `Opper`:
+
+* `Server` should be `https://api.opper.ai/v3/compat`
+* `API Key` should be an Opper API key, created at https://platform.opper.ai
+* A good starting `Model` is `gpt-5.4-mini`
+
 For `API Route`:
 
 * `Server` should be `https://global.api-route.com/v1`