fix: charge immediately on subscription and read item-level period dates

Three fixes to the Stripe subscription flow:

1. Change payment_behavior from 'default_incomplete' to
   'error_if_incomplete' so Stripe charges the card immediately instead
   of leaving the subscription in incomplete status waiting for frontend
   payment confirmation that never happens.

2. Read currentPeriodStart/End from subscription items instead of the
   top-level subscription object. Stripe moved these fields to
   items.data[0] in API version 2025-03-31.basil, causing epoch-zero
   dates (Dec 31, 1969).

3. Map Stripe 'incomplete' status to 'active' in mapStripeStatus() so
   it doesn't fall through to the default 'canceled' mapping.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

backend/src/features/subscriptions/domain/subscriptions.service.ts

@@ -570,11 +570,13 @@ export class SubscriptionsService {
     }
 
     // Update subscription with Stripe subscription ID
+    // Period dates moved from subscription to items in API 2025-03-31.basil
+    const item = stripeSubscription.items?.data?.[0];
     await this.repository.update(subscription.id, {
       stripeSubscriptionId: stripeSubscription.id,
       status: this.mapStripeStatus(stripeSubscription.status),
-      currentPeriodStart: new Date(stripeSubscription.current_period_start * 1000),
+      currentPeriodStart: new Date((item?.current_period_start ?? 0) * 1000),
-      currentPeriodEnd: new Date(stripeSubscription.current_period_end * 1000),
+      currentPeriodEnd: new Date((item?.current_period_end ?? 0) * 1000),
     });
 
     // Log event
@@ -608,11 +610,13 @@ export class SubscriptionsService {
     const tier = this.determineTierFromStripeSubscription(stripeSubscription);
 
     // Update subscription
+    // Period dates moved from subscription to items in API 2025-03-31.basil
+    const item = stripeSubscription.items?.data?.[0];
     const updateData: UpdateSubscriptionData = {
       status: this.mapStripeStatus(stripeSubscription.status),
       tier,
-      currentPeriodStart: new Date(stripeSubscription.current_period_start * 1000),
+      currentPeriodStart: new Date((item?.current_period_start ?? 0) * 1000),
-      currentPeriodEnd: new Date(stripeSubscription.current_period_end * 1000),
+      currentPeriodEnd: new Date((item?.current_period_end ?? 0) * 1000),
       cancelAtPeriodEnd: stripeSubscription.cancel_at_period_end || false,
     };
 
@@ -849,6 +853,7 @@ export class SubscriptionsService {
     switch (stripeStatus) {
       case 'active':
       case 'trialing':
+      case 'incomplete':
         return 'active';
       case 'past_due':
         return 'past_due';

backend/src/features/subscriptions/external/stripe/stripe.client.ts

@@ -86,7 +86,7 @@ export class StripeClient {
       const subscriptionParams: Stripe.SubscriptionCreateParams = {
         customer: customerId,
         items: [{ price: priceId }],
-        payment_behavior: 'default_incomplete',
+        payment_behavior: 'error_if_incomplete',
         payment_settings: {
           save_default_payment_method: 'on_subscription',
         },
@@ -101,13 +101,16 @@ export class StripeClient {
 
       logger.info('Stripe subscription created', { subscriptionId: subscription.id });
 
+      // Period dates moved from subscription to items in API 2025-03-31.basil
+      const item = subscription.items?.data?.[0];
+
       return {
         id: subscription.id,
         customer: subscription.customer as string,
         status: subscription.status as StripeSubscription['status'],
         items: subscription.items,
-        currentPeriodStart: (subscription as any).current_period_start || 0,
+        currentPeriodStart: item?.current_period_start ?? 0,
-        currentPeriodEnd: (subscription as any).current_period_end || 0,
+        currentPeriodEnd: item?.current_period_end ?? 0,
         cancelAtPeriodEnd: subscription.cancel_at_period_end,
         canceledAt: subscription.canceled_at || undefined,
         created: subscription.created,
@@ -148,13 +151,15 @@ export class StripeClient {
         logger.info('Stripe subscription canceled immediately', { subscriptionId });
       }
 
+      const item = subscription.items?.data?.[0];
+
       return {
         id: subscription.id,
         customer: subscription.customer as string,
         status: subscription.status as StripeSubscription['status'],
         items: subscription.items,
-        currentPeriodStart: (subscription as any).current_period_start || 0,
+        currentPeriodStart: item?.current_period_start ?? 0,
-        currentPeriodEnd: (subscription as any).current_period_end || 0,
+        currentPeriodEnd: item?.current_period_end ?? 0,
         cancelAtPeriodEnd: subscription.cancel_at_period_end,
         canceledAt: subscription.canceled_at || undefined,
         created: subscription.created,
@@ -294,14 +299,15 @@ export class StripeClient {
       logger.info('Retrieving Stripe subscription', { subscriptionId });
 
       const subscription = await this.stripe.subscriptions.retrieve(subscriptionId);
+      const item = subscription.items?.data?.[0];
 
       return {
         id: subscription.id,
         customer: subscription.customer as string,
         status: subscription.status as StripeSubscription['status'],
         items: subscription.items,
-        currentPeriodStart: (subscription as any).current_period_start || 0,
+        currentPeriodStart: item?.current_period_start ?? 0,
-        currentPeriodEnd: (subscription as any).current_period_end || 0,
+        currentPeriodEnd: item?.current_period_end ?? 0,
         cancelAtPeriodEnd: subscription.cancel_at_period_end,
         canceledAt: subscription.canceled_at || undefined,
         created: subscription.created,