feat: Implement subscription plan templates with dynamic pricing tier…

[Tekgodfrey]

…s (Issue #921)

Closes #921

Description

This PR integrates the Subscription Plan Templates and Dynamic Pricing Tiers logic, which was previously authored but unlinked across the backend and smart contract layers. Resolves Issue #921.

Key Changes

Smart Contract Integration (contracts/subscription)

• Types: Added TemplateKey enum and the PlanTemplate variant to StorageKey in contracts/types/src/lib.rs to resolve missing type errors.
• Contract Endpoints: Exported the plan_templates.rs module inside lib.rs and exposed the following core template functions in the SubTrackrSubscription contract block:
  • create_template
  • publish_template_version
  • set_template_shared
  • instantiate_template
  • Template query methods (get_template, get_owner_templates, get_shared_templates, etc.)
  • get_template_analytics
• Compiler Fixes: Resolved pre-existing compilation errors related to StorageKey and SorobanString in quota.rs, revenue.rs, and gas_storage.rs.

Backend Integration (backend/services/billing)

• Exports: Exported PlanTemplateService, its core methods (validateTiers, validateTemplateDraft, quoteTemplate, resolvePlan), and all associated TypeScript types (PricingTier, PlanTemplate, TemplateOverrides, ResolvedPlan, TemplateAnalytics) from index.ts.
• Dynamic Pricing: Verified that DynamicPricingStrategy was correctly exported and ready to be injected into the PricingStrategyFactory.

Testing & Validation

• Backend Tests: Verified test suite covers plan template CRUD, versioning, dynamic pricing tiers, and analytics via planTemplateService.test.ts. npm install was run to ensure dependencies are up-to-date and pass local test hooks.
• Contract Tests: The smart contract endpoints are structurally and syntactically integrated. (Note: Full cargo test execution requires resolving upstream soroban-sdk toolchain/workspace issues).

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