IA-965: feat: Trial period days, subscription status filter, and idempotent subscription upsert

MR Description

Summary

This MR introduces several improvements across the subscription and pricing plan modules.

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Changes

1. Trial Period Support

• Added trialPeriodDays field to PlanRequestDTO and PlanUpdateDTO
• Added getTrialPeriodDaysByGatewayPriceId() to PricingPlanService interface and PricingPlanServiceImpl, resolving trial days from the linked plan
• Exposed a new endpoint GET /pricing-plans/{gatewayPriceId}/trial-days in SubscriptionController

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2. Subscription Status Filtering

• Added getSubscriptionsByStatus() to SubscriptionService interface and SubscriptionServiceImpl, mapping SubscriptionStatusType to the internal StatusType
• Added findByStatus() to SubscriptionRepository
• Exposed a new endpoint GET /subscriptions?status= in SubscriptionController

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3. Idempotent Subscription Upsert

• createSubscription(SubscriptionRequestDTO) now performs an upsert: if the subscription already exists by gatewaySubscriptionId, it updates the status and period dates instead of failing or duplicating — with special logic to preserve TRIALING status against an incoming ACTIVE
• updateSubscription(String, SubscriptionRequestDTO) now creates the subscription if it doesn't exist, making the update endpoint resilient to out-of-order webhook delivery
• Added structured logging throughout both methods