Model failover: claude-code → anthropic → openrouter

Implement model failover chain for Ava.

Currently we have basic provider selection but no automatic failover. Moltbot has:

• Primary model + fallbacks list
• Auth profile rotation
• Cooldowns on failures
• Per-session model overrides

Implement: 1. Fallback chain: claude-code (primary) → anthropic API → openrouter 2. Automatic retry on rate limits or errors 3. Cooldown tracking (don't retry failed provider immediately) 4. Config in settings: model.primary, model.fallbacks

Reference: moltbot's model-failover.md and model handling