Integrate StoreKit for Premium Features

[DrunkOnJava]

📋 Sub-task of #97

Description

Implement StoreKit integration to enable premium features and in-app purchases. Currently, PurchaseService uses mock implementation.

Current State

• PremiumUpgradeView shows UI but no purchase functionality
• PurchaseService returns mock success
• No actual StoreKit integration
• Premium features inaccessible

Implementation Tasks

• Configure products in App Store Connect
• Implement StoreKitService conforming to PurchaseService
• Add product fetch and validation
• Implement purchase flow with receipts
• Add restore purchases functionality
• Implement subscription management
• Add receipt validation (server-side)

StoreKit Products

Products to create:
- com.homeinventory.premium.monthly (.99/month)
- com.homeinventory.premium.yearly (.99/year)
- com.homeinventory.premium.lifetime (.99)

Premium Features to Enable

• Unlimited items (vs 100 free)
• Advanced analytics
• Cloud backup
• Export to PDF/Excel
• Multiple homes/locations
• Barcode batch scanning

Technical Requirements

• StoreKit 2 implementation (iOS 15+)
• Async/await purchase flow
• Transaction observation
• Receipt validation
• Subscription status tracking
• Offline purchase queue

Testing Requirements

• Sandbox testing with test accounts
• Purchase flow testing
• Restore testing
• Subscription renewal testing
• Error handling testing

Files to Update

• Modules/Core/Sources/Services/PurchaseService.swift
• Modules/Premium/Sources/Views/PremiumUpgradeView.swift
• Create: Modules/Core/Sources/Services/StoreKitService.swift

Estimated Effort: 1 week

[DrunkOnJava]

Closing: This issue has been resolved/superseded by the complete modular architecture restructure now in main branch (commit 33a0ead). The new architecture implements a comprehensive solution that addresses the core needs this issue was tracking.