POC: fair queue, priority aware logic

[shaunwarman]

Fair Queue Implementation with Priority-Based Email Processing

Implements a fair queuing system for email processing with priority levels and
abuse detection:

Key Features:

• Priority Levels: HIGH (2), NORMAL (1), LOW (0), THROTTLED (-1)
• Fair Distribution: Domain and user-based fair queuing to prevent monopolization
• Adaptive Limits: Queue health monitoring with dynamic priority adjustments
• Abuse Detection: Automatic throttling based on bounce/reject rates and volume
  spikes
• Schema Updates: Added priority, abuse_score, and throttled_until fields

Components:

• Queue health calculation with adaptive multipliers
• Domain/user fair distribution algorithms
• Abuse score calculation and throttling logic
• Migration script for existing emails
• Enhanced monitoring and alerting

Benefits:

• Prevents single domains/users from monopolizing the queue
• Prioritizes premium users and good reputation senders
• Automatically throttles suspicious activity
• Maintains queue fairness under varying load conditions

[titanism]

Haven't had time yet to look at this but this POC looks promising at a glance.