Refactor subscription billing engine to strategy pattern for pluggable pricing

[Smartdevs17]

Context

Billing engine needs strategy pattern for different pricing models.

Current Limitation

Billing logic is monolithic with no pricing flexibility.

Expected Outcome

• Strategy pattern for pricing models
• Pluggable pricing strategies
• Pricing strategy testing
• Pricing configuration UI

Acceptance Criteria

• Pricing strategy interface
• Multiple pricing strategy implementations
• Strategy selection based on plan type
• Pricing strategy testing framework
• Pricing configuration UI
• Pricing analytics
• Pricing documentation

Technical Scope

• backend/services/billing/ - Billing domain
• contracts/subscription/src/ - Subscription contract
• src/store/subscriptionStore.ts - Subscription store
• src/screens/ - Pricing UI

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[Itodo-S]

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[Itodo-S]

Fixed in PR #828: #828

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