Feature Specification: Billing & Quotas

1. Overview & Vision

Billing & Quotas is the platform's monetization and resource-governance layer. It bridges the gap between commercial plans and technical limits, ensuring organizations are billed accurately for their usage while providing a seamless way to upgrade or manage subscriptions.

2. Personas & Stakeholders

Persona	Goal
Owner	Upgrade plans, manage payment methods, and view invoices.
Admin	Monitor organization usage vs. plan limits.
System Admin	Define global product tiers and monitor revenue.

3. User Stories

• As an owner, I want to upgrade to the "Pro" plan so I can add more than 5 members.
• As an admin, I want to see how much storage we have left in Drive before uploading large files.

4. Functional Requirements (FR)

• REQ-BILL-001: Support for subscription tiers (Hobby, Pro, Enterprise).
• REQ-BILL-002: Stripe Checkout integration for plan upgrades.
• REQ-BILL-003: Stripe Customer Portal for invoice and payment management.
• REQ-BILL-004: Real-time quota tracking (Storage, Users, AI Queries).

5. Non-Functional Requirements (NFR)

• Accuracy: Usage tracking must be atomic and eventually consistent within 1 minute.
• Security: Zero storage of raw credit card data (PCI compliance via Stripe).

6. Business Logic & Rules

• Hard vs. Soft Limits: Storage is a hard limit (stop uploads); AI queries are soft limits (metered overage).
• Grace Periods: 3-day grace period for failed payments before account locking.
• Pro-rated Billing: Stripe handles automated pro-ration for mid-cycle upgrades.

7. User Interface (UI/UX)

• "Plans & Pricing" comparison matrix.
• "Usage Dashboard" with progress bars for each quota.
• In-app banners for "Limit Approaching" or "Payment Failed" states.

8. Information Architecture

• "Billing" section within Organization Settings.
• Quick usage stats in the Shell sidebar (optional).

9. Data Model & Persistence

• Tables: subscription_plans, plan_limits, plan_usage.
• External: Stripe Products, Prices, and Customers.

10. API & Service Layer

• GET /api/subscriptions/usage
• POST /api/subscriptions/upgrade
• POST /api/subscriptions/webhook

11. Integration Patterns

• Stripe Webhooks: For syncing payment status and lifecycle events.
• Quota Hooks: Satellite modules increment usage counters in the Shell via internal API calls.

12. Security & Permissions

• Only organization Owner can access billing settings and checkout.
• Quota enforcement is checked at the API layer of each satellite module.

13. Error Handling & Resilience

• Graceful recovery from Stripe API timeouts.
• "Emergency Mode" (Read-only) when an account is locked for non-payment.

14. Performance & Scalability

• Quota counters are cached in Redis for high-frequency checks.
• Usage aggregation is performed asynchronously.

15. Globalization & i18n

• Support for multiple currencies (USD, VND).
• Region-specific tax calculation (delegated to Stripe).

16. Accessibility (a11y)

• Accessible pricing tables (aria-describedby for plan features).

17. Observability & Analytics

• Tracking of "Churn Rate" and "Upgrade Conversion" metrics.
• Monitoring of high-usage organizations for potential sales leads.

18. Testing & Quality

• Integration tests using Stripe CLI for webhook simulation.
• Unit tests for pro-ration and quota logic.

19. Constraints & Assumptions

• Assumes a valid Stripe account is configured in the environment.

20. Future Enhancements

• Usage-based (metered) billing for AI tokens.
• Add-on resource packs (e.g., "+10GB Storage").