feat: implement subscription plan templates with dynamic pricing tiers

[Timmatt112]

Summary

Closes Smartdevs17#950

Adds production-facing subscription plan template support to the Soroban subscription implementation, including reusable template storage, graduated dynamic pricing tiers, versioning, sharing, instantiation, and analytics.

Changes

• Added append-only TemplateKey and StorageKey::PlanTemplate variants for template persistence.
• Registered the existing plan_templates module in the subscription contract.
• Exposed authenticated contract entrypoints for creating templates, publishing versions, sharing, instantiation, tier quotes, and analytics.
• Preserved prior storage-key ordering for upgrade compatibility.
• Reused the existing backend template service, tier calculator, and comprehensive tests.

Dynamic pricing behavior

• Supports up to 12 ascending tiers.
• Supports included/free units via zero-priced tiers.
• Supports a final unbounded tier.
• Rejects negative prices, invalid bounds, and non-terminal unbounded tiers.
• Calculates graduated pricing by charging each unit only within its applicable tier.

Validation

• git diff --check passed.
• Formatting and backend plan-template tests were attempted.
• Full verification is blocked by pre-existing repository issues:
  • Rust/Cargo is unavailable in the environment.
  • TypeScript has unrelated merge-conflict/syntax errors.
  • Backend Jest initialization fails on a pre-existing type-only ErrorCode runtime reference.

Scope hygiene

The four pre-existing modified generated/dependency files were intentionally excluded:

• package-lock.json
• src/contracts/types/ERC20.ts
• src/contracts/types/common.ts
• src/contracts/types/factories/ERC20__factory.ts