feat: License Key System + Anti-Piracy Validation

[Delqhi]

Objective

Implement license key generation, validation, and anti-piracy measures.

Requirements

• Generate unique license keys on subscription activation
• Email license key to user via Stripe/Supabase
• Validate license key on every API request (server-side)
• Device fingerprinting (max 3 devices per license)
• License key revocation on subscription cancellation
• Grace period: 3 days after payment failure before deactivation
• Rate limit by license key (not just user ID)

Anti-Piracy Measures

• License validation is ALWAYS server-side (never trust client)
• Extension checks subscription status on every startup
• Short-lived JWTs (15min) prevent token sharing
• Device limit prevents credential sharing
• Usage anomaly detection (too many requests = review)

Acceptance Criteria

• License keys generated automatically on payment
• Server rejects requests with invalid/expired keys
• Device limit enforced
• Grace period works correctly