Refactor subscription billing engine to strategy pattern for pluggable pricing

[Smartdevs17]

Context

The billing engine uses a large switch-case for pricing model selection, making it difficult to extend or test individual pricing strategies.

Current Limitation/Problem

Adding a new pricing model (tiered usage, hybrid, custom) requires modifying the core billing engine file, risking regressions across all pricing models.

Expected Outcome

Strategy pattern: each pricing model is a separate class implementing a common PricingStrategy interface, registered in a dynamic strategy registry.

Acceptance Criteria

• PricingStrategy interface: calculate(usage, plan, subscriber) -> Amount
• Extract FlatPricingStrategy, PerSeatPricingStrategy, UsageBasedPricingStrategy, TieredPricingStrategy
• Strategy registry with dynamic lookup by plan type code
• All existing billing tests pass without modification
• New strategy can be added as a single file without touching existing code
• Performance: no regression in billing calculation time (measured <5ms per invoice)
• Edge case: fallback strategy for unknown/unsupported plan types

Technical Scope

• backend/billing/domain/strategies/ - individual strategy class files
• backend/billing/domain/ - BillingEngine refactored to delegate to strategy
• backend/billing/domain/ - PricingStrategy interface and StrategyRegistry
• backend/billing/tests/ - strategy-specific test files with >90% coverage

[topboy192]

@topboy192 has applied to work on this issue as part of the Stellar Wave Program's 6th wave.

i will like to work on this issue

ℹ️ Repo Maintainers: To accept this application, review their application or assign @topboy192 to this issue.

[barry01-hash]

@barry01-hash has applied to work on this issue as part of the Stellar Wave Program's 6th wave.

I will love to work on this issue

ℹ️ Repo Maintainers: To accept this application, review their application or assign @barry01-hash to this issue.

[drips-wave]

Congratulations, @barry01-hash! 🎉 Your application was accepted by the repo's maintainers, and the issue is due on June 30, 2026.

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