Add send rate limiting/queuing to prevent WhatsApp bans

Queue outgoing messages with rate limiting (~10 messages/sec) to prevent triggering WhatsApp's anti-abuse mechanisms.

**Why:** WhatsApp has undocumented rate limits - too fast = temporary ban or disconnect.

**When needed:** Only relevant if sending >100 messages/minute via automation.

**Effort:** ~1 hour  
**Priority:** Medium value (only needed at scale)