audio: configuration section and documentation

[bug-ops]

Parent: #520
Depends on: #522, #523, #524, #525, #526

Context

Document audio input configuration and update mdbook docs.

Changes

config/default.toml

[audio]
enabled = false
backend = "whisper-api"  # whisper-api | whisper-local | none
language = "auto"
max_duration_secs = 120
max_file_bytes = 26214400  # 25 MB

[audio.whisper_api]
model = "whisper-1"

[audio.whisper_local]
model = "whisper-base"
device = "auto"

Documentation

• New page: `docs/src/guide/audio.md`
• Update SUMMARY.md
• Update README.md with audio mention
• Update channel guides with audio examples

Environment variables

• `ZEPH_AUDIO_ENABLED`
• `ZEPH_AUDIO_BACKEND`
• `ZEPH_AUDIO_LANGUAGE`

Acceptance criteria

• Config section in default.toml
• mdbook page with setup instructions per backend
• Channel-specific audio examples
• Feature flags documented