- Remove legacy TTS rate limit environment variables and migrate all related configuration to runtime settings managed via the Admin UI
- Add runtime config keys for TTS rate limiting enable/disable and per-user/IP daily quotas
- Refactor rate limiter and API routes to use runtime config for thresholds and enablement
- Update AdminFeaturesPanel to allow editing TTS rate limiting and quota values in the UI
- Add shared-provider-selection utility for consistent provider selection logic
- Update documentation to reflect new runtime/admin configuration and remove obsolete env var docs
- Add unit tests for rate limit runtime settings and provider selection
BREAKING CHANGE: TTS rate limiting is now controlled via Admin → Site features; environment variables for TTS rate limiting are no longer supported and will be ignored.
