Adds usage tracking to auth profiles for automatic rotation:
- ProfileUsageStats type with lastUsed, cooldownUntil, errorCount
- markAuthProfileUsed(): tracks successful usage, resets errors
- markAuthProfileCooldown(): applies exponential backoff (1/5/25/60min)
- isProfileInCooldown(): checks if profile should be skipped
- orderProfilesByMode(): now sorts by lastUsed (oldest first)
On auth/rate-limit failures, profiles are marked for cooldown before
rotation. On success, usage is recorded for round-robin ordering.
This enables automatic load distribution across multiple accounts
(e.g., Antigravity 5-hour rate limit windows).
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