v3.8.1 — docs-only sync for v3.8.0

README + docs/*.md + .env.example catch up to the talkies v0.9.0
model set (6 CPU models / 14 CUDA models). No behaviour change, no
image-pin change, no LiteLLM config change — the actual upgrade
shipped in v3.8.0.

Updated files:
  - README.md (services overview table — talkies CPU + CUDA rows)
  - docs/providers.md (talkies CPU + CUDA model tables)
  - docs/services-reference.md (talkies model bullet list + Qwen3-TTS
    family sub-section documenting mode-implicit voice / instructions
    semantics, PCM streaming, sampling controls)
  - docs/usage.md (transcription + speech examples + model bullets)
  - docs/testing.md (model-count assertion line)
  - .env.example (TALKIES / TALKIES_CUDA doc comments +
    TALKIES_QWEN3_STREAM_CHUNK_SIZE knob)

See CHANGELOG.md for the full per-file breakdown.