stt: uniform hotwords support across ASR backends (#1781 part one)

Each supporting ASR model now accepts a uniform hotwords: list[str] in its own
generate() and folds it into its native prompt field: qwen3_asr->system_prompt,
whisper->initial_prompt, vibevoice_asr->context, moss_transcribe_diarize->prompt.
fun_asr_nano already handles hotwords natively. Backends without a hook ignore
the field (silent drop, per maintainer preference). Shared merge_hotwords helper
in stt/utils.py; models keep their native fields for backward compatibility.