feat(providers): an OpenAI-compatible preset for any vLLM, LM Studio or gateway URL

Step 8 of the provider refactor plan, a new provider made of one record.
'custom' runs on @ai-sdk/openai-compatible with <think> extraction, merges
system messages (open-source chat templates reject several), has no default
URL (so a key alone does not count as connected, and the chat endpoint is
shown under the URL field) and lists models from its /models. Server side:
CUSTOM_API_KEY and CUSTOM_BASE_URL, ADMIN_CUSTOM_* in the admin panel.
env.example and the three provider docs gained a section; the picker count
in the e2e test is 25 and a new e2e test walks through the preset. The
behavior snapshot gained the new provider's 146 rows; its two 'No AI provider
configured' rows list CUSTOM_API_KEY now.

docs/en/ai-providers.md

@@ -338,6 +338,18 @@ Optional custom endpoint (Token Plan subscribers should set their dedicated Base
 MIMO_BASE_URL=https://token-plan-cn.xiaomimimo.com/v1
 ```
 
+### Any OpenAI-compatible API (custom)
+
+For a server not listed above (vLLM, LM Studio, LiteLLM, an API gateway), pick **OpenAI-compatible** in the settings and enter its base URL and a key; a server that ignores keys takes any non-empty value. Several system messages are merged into one, as open-source chat templates expect, and `<think>` reasoning is split from the reply.
+
+```bash
+CUSTOM_API_KEY=your_api_key
+CUSTOM_BASE_URL=http://127.0.0.1:8000/v1
+AI_MODEL=your_model_id
+```
+
+---
+
 ## Auto-Detection
 
 If you only configure **one** provider's API key, the system will automatically detect and use that provider. No need to set `AI_PROVIDER`.