Fixes#2816
Changed OPENAI_TTS_BASE_URL from a module-level constant to a function
that reads process.env at call time. This allows config.env settings to
take effect after gateway startup.
Changes:
- OPENAI_TTS_BASE_URL constant → getOpenAITtsBaseUrl() function
- isCustomOpenAIEndpoint constant → isCustomOpenAIEndpoint() function
- Updated 3 call sites to use function calls
Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>
