feat(rtzr): add keyword boosting to streaming STT

[lalq]

Description

Added RTZR keyword boosting support by exposing keywords in the STT interface and passing it to the streaming config.
Implemented keyword serialization/validation for WebSocket (keyword or keyword:score, (keyword, score) tuples).
Documented usage, limits, and Hangul-only requirement in plugin README.

Changes

rtzrapi.py
stt.py
README.md

Notes

Keyword boosting is only supported for sommers_ko and requires Hangul-only keywords; non-Korean input errors.

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