feat: rate-limit Plivo audio send to ~1.2x real-time

[san0808]

Summary

• TTS generates audio much faster than real-time, causing 60s+ buffer buildup in Plivo. Mark ACKs get delayed, hangup timeout expires, thus calls cut off mid-sentence.
• Adds rate-limiting to TelephonyOutputHandler base class (same algorithm as SIP-Trunk handler), gated on io_provider == 'plivo'
• Caps send rate at 1.2x real-time (configurable via TELEPHONY_MAX_SEND_RATE_FACTOR env var, 0 to disable)
• Resets rate-limit counters on sequence change and interruption
• 60s response now builds max ~12s buffer (well within Plivo's 40s limit)