feat: api accepts 'vllm' as a first-class value

'openai' reads as the cloud company — an operator running
self-hosted vLLM should be able to write what they mean. The new
value speaks the identical OpenAI wire (a guard test asserts the
emitted request stays byte-identical to api: openai, so the alias
can never drift into a dialect) but yields vLLM-specific error
hints (vllm serve, port 8000) instead of generic OpenAI prose.
Manifest + docs name the value in DE and EN.

Signed-off-by: flemming-it <sf@flemming.it>

MODULE.md

@@ -17,7 +17,7 @@ model-provenance fields.
 | `target_language` | text | Target language in plain English (e.g. `German`, `French`, `ja-JP`).   |
 | `source_language` | text | Optional source-language hint. Empty = let the model auto-detect.      |
 | `endpoint`        | text | Chat endpoint URL matching the selected `api` (Ollama `/api/chat`, OpenAI-compatible `/v1/chat/completions` — vLLM etc., Anthropic `/v1/messages`). |
-| `api`             | text | Optional wire format: `ollama` (default), `openai`, `anthropic`.        |
+| `api`             | text | Optional wire format: `ollama` (default), `vllm`, `openai`, `anthropic` (`vllm` = the OpenAI wire with vLLM-specific hints).        |
 | `model`           | text | Model identifier the endpoint serves (e.g. `qwen2.5:14b`).             |
 | `api_key`         | text | Optional bearer token for cloud-hosted endpoints.                      |