test(subscription): add unit tests for initialize and admin paths (#890)

[had3sgames]

Add tests for PlanCount=0 on init, initialized event emission, admin() panic on uninitialized contract, and admin address stability across fee configuration changes.

Summary

Changes

Test Plan

Automated tests added or updated

• Unit tests (backend/src/**/*.spec.ts) — service/guard/decorator logic in isolation
• Integration / e2e tests (backend/test/**/*.e2e-spec.ts) — HTTP round-trips with mocked infrastructure
• Frontend component tests (frontend/src/**/*.test.{ts,tsx}) — React component behaviour
• Frontend e2e tests (frontend/e2e/**/*.spec.ts) — Playwright browser flows
• Contract tests (contract/) — Soroban/Rust unit tests via cargo test
• No new tests required — explain why: ___

How to run the tests locally

# Backend unit tests
cd backend && npm test

# Backend e2e tests (requires no live DB — uses in-memory mocks)
cd backend && npm run test:e2e

# Frontend component tests
cd frontend && npx vitest run

# Frontend e2e tests (requires dev server on :3000 and API on :3001)
cd frontend && npx playwright test

# Contract tests
cd contract && cargo test

Manual verification checklist

• Happy path works end-to-end in a local environment
• Error / edge cases handled gracefully (stale state, invalid input, disconnected wallet)
• No regressions in closely related API or UI flows
• Rate-limiting, auth guards, and feature flags behave as expected where touched
• Linting passes: cd backend && npm run lint / cd frontend && npm run lint

Related issues

Notes for reviewers