Add ClassifierBackend (NER/PII via transformers token-classification pipeline)
and TextFilter (redact / detect / spans modes). MockClassifierBackend provides
deterministic PII spans for tests and CI without GPU. Enables privacy-safe
pre-screening before LLM inference.
Adds POST /v1/chat/completions to the cf-text FastAPI service so it can
be used as an openai_compat backend in LLMRouter without any router changes.
The endpoint accepts the standard OpenAI chat request format and returns
a standard chat.completion response.
4 tests added; all 36 text tests pass.