[Feature]: Account pool round-robin / fill-first rotation strategies

[Wibias]

Summary

Operators want explicit account-pool load spreading (round-robin / fill-first) in addition to today's quota-threshold routing, without breaking thread affinity.

Acceptance

• Codex pool strategies: quota (default) | round-robin | fill-first
• Anthropic pool parity when enabled
• Affinity preserved for live threads
• Docs + dashboard controls
• Tests for fairness + affinity

[github-actions]

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