Added 60dB integration

[manishEMS47]

We added 60db as a second, switchable TTS provider alongside ElevenLabs.

• src/lib/server/tts.ts (new) — provider layer; synthesizeSpeech() routes to ElevenLabs or 60db based on the TTS_PROVIDER env var, both returning the same shape.
• src/routes/api/tts/+server.ts — slimmed to call the dispatcher.
• src/lib/audio/tts.ts — added amplitude-envelope lip-sync so 60db (which returns no timing data) still animates the mouth;
  ElevenLabs keeps phoneme-accurate sync.
• README.md — documented the env vars, how to switch providers, and how each one drives lip-sync.

Switch with TTS_PROVIDER=elevenlabs or TTS_PROVIDER=60db + the matching API key. No UI/STT/streaming added (per your choices).

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