feat: refactor testing infrastructure to use test containers and fixtures (#976)

- Rewrite backend/tests/setup/testContainer.ts with full production-ready
  test container infrastructure:
  * TestContainerManager: full lifecycle (start/stop), in-memory entity store
    with seedDatabase, cleanDatabase, createSavepoint/rollbackToSavepoint for
    per-test isolation, named takeSnapshot/restoreSnapshot, findById/findAll/
    findWhere/upsert/delete/count query helpers
  * FixtureLoader: typed factories for all core domain entities (plan, user,
    subscription, merchant, invoice) with auto-incrementing IDs, sensible
    defaults, and fullSubscriptionScenario() for complete linked graphs
  * DatabaseSeeder: fluent builder that inserts in dependency order
    (merchants → plans → users → subscriptions → invoices)
  * TestClock: deterministic time control with advance/advanceHours/advanceDays
    /set/reset for time-sensitive tests
  * TestEventBus: in-memory event capture with assertPublished/assertNotPublished
    /ofType/last/count for integration assertions without real message brokers
  * RedisFixture: tracks keys for automatic cleanup, supports pattern cleanup
  * detectFlakyTest: retry-based flakiness detection with pass/fail counts and
    error messages
  * withTiming + assertWithinMs: performance assertions with configurable limits
  * createTestContext: convenience factory wiring all components together
  * Exports testContainerManager singleton for backward compatibility

- Expand backend/tests/integration/testContainer.test.ts from 3 smoke tests
  to 70+ cases covering all components:
  * Lifecycle: start/stop idempotency, initialSeed, data cleared on restart
  * Seeding: findById, findAll, findWhere, upsert, delete, count,
    cleanDatabase (targeted and full), error on missing id
  * Savepoints: rollback restores state, stacking works correctly
  * Snapshots: take/restore/getSnapshot, error on unknown id
  * FixtureLoader: all factories, overrides, unique IDs, fullSubscriptionScenario
  * DatabaseSeeder: dependency ordering, chaining, withRaw
  * detectFlakyTest: always-pass, always-fail, intermittent, error capture
  * TestClock: advance/hours/days, set, date, iso, reset, chaining
  * TestEventBus: publish/ofType/last/count/assert/clear, copy isolation
  * RedisFixture: set/get, cleanup, cleanupPattern, trackedCount
  * withTiming/assertWithinMs: result passthrough, pass/fail cases
  * createTestContext: full lifecycle smoke test

- Update jest.backend.config.js to include backend/tests/**/*.test.ts and
  backend/billing/tests/**/*.{test,spec}.ts in testMatch; exclude load/ dir

- Update docs/integration-tests.md with full API reference, usage examples,
  per-test isolation pattern, and performance benchmarks

Closes #976

Author: Clement-coder