feat(tts): add Google Cloud TTS provider and markdown stripping

[georgernstgraf]

Builds on #63 — adds an alternative TTS backend and ensures clean speech output.

Summary

• Google Cloud TTS provider: New TTS_PROVIDER=google option alongside the existing OpenAI-compatible path. Uses @google-cloud/text-to-speech SDK with a singleton client. Language code is auto-extracted from the voice name (e.g. de-DE from de-DE-Neural2-B), so BOT_LOCALE doesn't need to match.
• Markdown stripping: All text is cleaned before synthesis — removes **bold**, *italic*, `code`, fenced code blocks, links, headings, list markers, HTML tags, etc. This prevents TTS engines from reading markdown syntax aloud (asterisks, backticks, etc.).
• 20 new tests: Covers markdown stripping, language code extraction, Google provider detection, and markdown-stripped synthesis. All 713 tests pass.
• Updated .env.example: Includes Google Cloud setup instructions and free tier info (1M chars/month for Neural2/WaveNet, 4M/month for Standard).

Config

# OpenAI-compatible (default, no change for existing users)
TTS_PROVIDER=openai
TTS_API_URL=https://api.openai.com/v1
TTS_API_KEY=sk-...
TTS_VOICE=alloy

# Google Cloud TTS
TTS_PROVIDER=google
TTS_VOICE=en-US-Studio-O
GOOGLE_APPLICATION_CREDENTIALS=/path/to/key.json

Backward compatibility

Fully backward-compatible. Default provider is "openai", so existing deployments continue to work without any changes. The stripMarkdownForSpeech function applies to both providers.

Closes #63

[Copilot]

Pull request overview

Adds a Google Cloud Text-to-Speech backend option and ensures TTS input is cleaned of Markdown so spoken output is natural.

Changes:

• Added TTS_PROVIDER config with a new google provider implemented via @google-cloud/text-to-speech.
• Introduced stripMarkdownForSpeech() to remove common Markdown/HTML markers before synthesis (applies to all providers).
• Expanded Vitest coverage for markdown stripping, language code extraction, and provider configuration detection.

Reviewed changes

Copilot reviewed 5 out of 6 changed files in this pull request and generated 7 comments.

Show a summary per file

File	Description
src/tts/client.ts	Adds Google TTS provider implementation, markdown stripping, and provider routing logic.
src/config.ts	Adds tts.provider config sourced from TTS_PROVIDER.
tests/tts/client.test.ts	Adds tests for provider config detection, markdown stripping, and language code extraction.
package.json	Adds @google-cloud/text-to-speech dependency.
package-lock.json	Locks new Google Cloud dependency tree.
.env.example	Documents TTS_PROVIDER=google setup and Google credentials configuration.

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[georgernstgraf]

works very fine for me and its on a free tier.

I did my best to resolve copilots (important) comments.

I did not manually test if openai TTS still works.

The markdown stripping does a quite good job for me, but it is still far from perfect.
Currently the voices leave out pauses where some belong. Want to investigate further.

planned by me & glm-5.1, coded by glm-5.1, reviewed by me ;)

This is such an awesome setup, thank you for this great repo!

[grinev]

@georgernstgraf thanks for the PR! The overall direction looks good, but could you please address a few issues before we merge?

1. For Google TTS, the default voice currently still resolves to alloy from config. This means TTS_PROVIDER=google without TTS_VOICE will likely send alloy to Google and fail. Please use a Google-specific default voice, for example en-US-Studio-O.

2. extractLanguageCode() only supports voice names like de-DE-. Some Google voices use 3-letter language codes, for example cmn-CN- or yue-HK-*. Please update the extraction logic and add tests for these cases.

3. isTtsConfigured() always returns true for Google. This can let users enable /tts even when Google credentials are not actually available. Please either check GOOGLE_APPLICATION_CREDENTIALS or document/handle ADC clearly so users do not get repeated synthesis failures at runtime.

[grinev]

@georgernstgraf thanks for the new feature contribution!