feat(copilot): add billing endpoint

[Sg312]

Summary

Adds update billing endpoint for agent

Type of Change

• New feature

Testing

Manually

Checklist

• Code follows project style guidelines
• Self-reviewed my changes
• Tests added/updated and passing
• No new warnings introduced
• I confirm that I have read and agree to the terms outlined in the [Contributor License Agreement (CLA)

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[greptile-apps]

Greptile Summary

This PR introduces copilot-specific billing functionality by adding a new cost multiplier for copilot operations and creating an internal billing endpoint. The changes consist of three main components:

1. Environment Configuration: A new optional environment variable COPILOT_COST_MULTIPLIER is added to lib/env.ts with proper Zod validation, following the existing pattern of the COST_MULTIPLIER variable.

2. Cost Calculation Enhancement: The calculateCost function in providers/utils.ts is extended with an optional customMultiplier parameter, allowing callers to override the default cost multiplier while maintaining backward compatibility through the nullish coalescing operator.

3. New Billing Endpoint: A new route at /api/billing/update-cost provides an internal API for external services to report copilot token usage. The endpoint features internal API key authentication, comprehensive input validation with Zod schemas, and database operations that mirror the existing ExecutionLogger patterns.

The implementation enables separate cost tracking for copilot operations versus regular workflow executions, allowing administrators fine-grained control over copilot pricing. The architecture maintains consistency with existing billing infrastructure while providing the flexibility needed for different product features to have distinct cost structures.

Confidence score: 4/5

• This PR introduces new billing functionality with minimal risk to existing systems due to backward compatibility
• Score reflects well-structured implementation following existing patterns, though the new billing endpoint requires careful monitoring
• Pay close attention to the new billing endpoint route.ts for proper authentication and cost calculation logic

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