[Testing 08] test: route-level integration tests + real-Supabase auth/RLS suites

Ports the backend route-level integration suites from the amal66 fork
(index: Open-Legal-Products/mike#205), adapted from the fork's
apps/api modules/services layout to this repo's monolithic
backend/src/routes/*.ts layout.

app/index split (the only production change; mechanical, zero behavior
change): backend/src/index.ts previously built the express app and
called app.listen at the bottom. Everything except the listen call
moved verbatim into backend/src/app.ts, which now exports `app` (same
middleware order, same routes, same rate-limiter setup, dotenv/config
still imported first). index.ts is now a tiny entry that imports
{ app } and calls listen with the same log line. `npm run build` still
emits dist/index.js and the `start` script is unchanged.

New suites under backend/src/__tests__/integration/ (94 tests):
- health.test.ts (5): /health, requireAuth 401 paths, 404 fallthrough
- chat.routes.test.ts (6): POST /chat validation + SSE happy/error paths
- projects.routes.test.ts (18): overview/create/detail/patch/delete,
  sharing normalisation, ownership guards
- projectChat.routes.test.ts (3): project access guard + SSE paths
- tabular.routes.test.ts (31): review CRUD, access guards,
  document-access filtering, missing_api_key guards
- user.routes.test.ts (27): profile, API-key crypto boundary, MFA
  guards, export/deletion endpoints
- documentsUpload.routes.test.ts (4): upload validation + download-zip
  bounds/access
- access.supabase.test.ts (1) + stack.supabase.test.ts (4): gated on
  SUPABASE_TEST_URL / SUPABASE_TEST_SERVICE_ROLE_KEY (stack suite also
  needs SUPABASE_TEST_ANON_KEY); describe.skip otherwise
- scripts/test-stack.sh + `npm run test:stack`: reads a running
  `supabase status -o json` and runs the gated suites

Adds supertest + @types/supertest as devDependencies.

Dropped relative to the fork (subjects that do not exist in this repo):
- orgs.routes, credits.concurrency.supabase, dmsConnectors suites
  (fork-only features)
- all credit-reservation cases (429 CREDIT_LIMIT_EXCEEDED,
  reserve-then-refund) in chat/projectChat: no credit system here
- health /ready case: no /ready endpoint here
- org-membership project access case: no org model here
- upload magic-byte validation cases: this repo validates extension
  only (the fork adds content sniffing; replaced with a missing-file
  400 case)
- download-zip 50-document cap case: no cap here (replaced with a
  no-accessible-documents 404 case)
- stack.supabase PUBLIC_TABLES updated to this repo's schema; the
  fork's credit-RPC coverage is n/a

Verified locally: backend `npm test` -> 8 files passed, 2 skipped;
106 tests passed, 5 skipped (gated suites skip without env).
`npm run build` passes. The gated suites were additionally run against
a live local Supabase stack: 2 files, 5/5 tests passed.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

The "Merge branch 'main'" commit (71a7aba) auto-merged backend/package.json
and backend/package-lock.json without raising a conflict, but git's
line-level merge produced invalid JSON in both files: package.json lost the
comma after the new "test:stack" script, and package-lock.json lost the two
closing-brace lines of the supertest/cookie-signature entry. npm treats an
unparseable lockfile as absent, which is why CI failed with the misleading
"npm ci can only install with an existing package-lock.json".

package.json: restore the comma. package-lock.json: rebuilt from main's
known-good copy via `npm install --package-lock-only` against this branch's
package.json. Verified locally: npm ci, npm test (259 passed), npm run build.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>