feat: implement subscription plan templates with dynamic pricing tiers

[Timmatt112]

Summary

Closes Smartdevs17#950

Adds reusable subscription plan templates with dynamic graduated pricing tiers across the subscription contract and billing domain.

What changed

• Added namespaced on-chain template storage keys and template lifecycle support.
• Exposed contract APIs for creating, versioning, sharing, instantiating, quoting, listing, and analyzing templates.
• Added safeguards for tier definitions and feature limits.
• Prevented tiered templates from bypassing their pricing ladder via a flat price override.
• Exported the backend template service and helpers from the billing module.
• Added focused coverage for the tier-price override behavior.

Pricing behavior

Tiered templates use graduated pricing: each tier charges only the units inside its range. An optional final unbounded tier handles usage above the last bounded threshold. Flat templates continue to use their base price regardless of usage.

Compatibility and safety

• Existing plan creation remains unchanged.
• Template-created plans resolve into ordinary Plan records.
• Superseded versions remain readable but cannot be instantiated.
• Shared templates are instantiable by other merchants but remain owner-editable only.
• New storage enum variants are appended to preserve existing key discriminants.

Verification

• git diff --check passed.
• Focused TypeScript formatting/lint hooks passed during commit.
• Full typecheck is blocked by pre-existing errors in unrelated notification, screen, and fraud files.
• Focused billing tests are blocked before execution by a pre-existing ErrorCode type/value issue in backend/services/shared/errors.ts.
• Contract tests could not run because Rust/Cargo is unavailable in the environment.

Scope note

Pre-existing generated typings and package-lock.json modifications were intentionally left unstaged and are not part of this PR.