feat(billing): integrate Loops checkout for subscription upgrades

[heyfuaad]

Summary

• Add Loops SDK (@loops-fi/sdk) for checkout session management
• Create Loops client utility with environment variable support
• Implement Loops checkout helper using checkoutSessions API
• Add custom checkout API endpoint (/api/billing/checkout)
• Update subscription upgrade flow to use Loops checkout when enabled
• Fallback to Stripe checkout if Loops is not configured
• Use existing Stripe price IDs directly in Loops checkout sessions
• Add LOOPS_API_KEY environment variable

This integration allows using Loops checkout links while keeping all other billing functionality (webhooks, subscriptions) on Stripe.

Type of Change

• [ ] Bug fix
• [ ] New feature
• [ ] Breaking change
• [ ] Documentation
• [ ] Other: ___________

Testing

How has this been tested? What should reviewers focus on?

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)

Screenshots/Videos

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