fix(backend): restore real auth.js exports in test mocks so suites load

[ogazboiz]

Why

main is red again on the Backend CI job. PR #703 (swagger-docs-auth-tests) rewrote the auth.js mock in the five stream route test files to a plain factory:

vi.mock('../src/middleware/auth.js', () => ({
  requireAuth: ...,
  requireAdmin: ...,
}));

auth.js also exports issueChallenge, verifyChallenge and verifyJwt, which auth.routes wires up when the app is constructed. With those exports missing from the mock, importing src/app.js throws:

[vitest] No "issueChallenge" export is defined on the "../src/middleware/auth.js" mock

so all five suites fail to load — CI reports 5 failed | 31 passed, 146 tests run instead of 184, and the coverage step is skipped.

Fix

Use vi.mock with importOriginal to spread the module's real exports and override only requireAuth/requireAdmin. The app loads, the middleware is still stubbed for the tests, and the suites run. (This is the pattern from #625; #703 replaced it with a comment claiming importOriginal was unreliable with pool: forks, but it runs fine here and in CI.)

Affected files:

• backend/tests/stream.test.ts
• backend/tests/integration/streams.test.ts
• backend/tests/integration/top-up.test.ts
• backend/tests/integration/streams/withdraw.test.ts
• backend/tests/integration/streams/cancel.test.ts

Verification (local, against Postgres)

• Test Files 36 passed (36)
• Tests 184 passed (184)
• Coverage: 70.15% stmts / 66.73% branches / 72.34% funcs / 70.15% lines — all above the 60% gate.

Frontend and contracts jobs are unaffected (already green on this main).