Add a basic test harness and HTTP smoke tests

[CryptoJones]

Problem

The repository has zero tests. There is no way to verify that a refactor (for example, the env-var work in the configurable-port-and-CORS issue) hasn't broken the API contract, and no regression test pinned to the existing auth bug (#3).

Scope

• Add a test runner (vitest or jest).
• Add a small supertest-based suite covering at minimum:
  • GET /v1/customer/:id — 200 on a valid record, 404 on missing, 401 (or whatever the spec says) when authKey is missing.
  • GET /v1/customer/bycompany/:id — equivalent shape coverage, including a regression test pinned to the auth bug fix from Broken Authentication in GET endpoint /v1/customer/bycompany/# #3.
• Run tests against a local Postgres seeded from setup/test_seed.sql, or stub Sequelize for unit-level coverage.
• Wire npm test to invoke the suite.

Acceptance criteria

• npm test runs and exits 0 on a healthy checkout.
• At least 4 distinct test cases across the two existing endpoints.
• One regression test pinned to the behavior fix from Broken Authentication in GET endpoint /v1/customer/bycompany/# #3.

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