1. Subscription Group Configuration

• 1.1 Create SubscriptionGroup enum with AI, VC, Care groups
• 1.2 Create SubscriptionPlanPriority configuration with plan-to-priority mapping
• 1.3 Implement SubscriptionGroupService to identify plan groups and priorities
• 1.4 Add Stripe price ID to plan mapping configuration

2. Upgrade Implementation

• 2.1 Add upgrade detection logic in StripePayService
• 2.2 Implement upgrade API call to Stripe (subscription update with proration)
• 2.3 Add proration preview endpoint support
• 2.4 Implement immediate plan benefit activation after upgrade
• 2.5 Add trial period check to block upgrades during trial

3. Downgrade Implementation

• 3.1 Add downgrade detection logic in StripePayService
• 3.2 Implement downgrade API call to Stripe (subscription update without proration)
• 3.3 Implement scheduled downgrade at next billing cycle
• 3.4 Add AI Family downgrade device rebinding logic
• 3.5 Handle Care downgrade without device requirements

4. Confirmation Dialog Handling

• 4.1 Add plan change confirmation API endpoint
• 4.2 Implement upgrade confirmation response with proper messaging
• 4.3 Implement downgrade confirmation response with next billing date
• 4.4 Add button debounce (1.5s) on frontend (coordinated with mobile team)

5. Webhook Processing

• 5.1 Add customer.subscription.updated webhook handler
• 5.2 Implement subscription status sync logic
• 5.3 Handle device rebinding when downgrade takes effect
• 5.4 Add webhook idempotency checks

6. Existing Subscription Check

• 6.1 Implement check for existing subscription in same group
• 6.2 Add error message for duplicate plan purchase attempt
• 6.3 Add validation for same-plan selection

7. Testing

• 7.1 Write unit tests for subscription group priority logic
• 7.2 Write unit tests for upgrade/downgrade detection
• 7.3 Write integration tests for Stripe API calls
• 7.4 Test webhook processing scenarios