feat(eims): add invoice mapper and signed EIMS transport

Map EDR invoices onto the MoR EIMS /v1/register document and add the
cryptographic transport needed to talk to core.mor.gov.et.

Mapper: DTOs mirror the supplied Postman collection section by section.
Tax is resolved per line via a caller-supplied resolver and throws when
unresolved -- the app models no tax at all (invoice.taxAmount is always 0,
invoice_lines and the rate catalogue carry no fiscal columns), so a
zero-rated default would assert a tax position the codebase cannot support.
Seller identity, document number, counters and previous IRN are passed in
explicitly; the mapper stays pure.

Transport: config, credential loading, RSA-SHA512 signing and /auth/login
with an in-memory token cache. Signing reproduces the process that produced
a working live token -- compact JSON of the inner request only, exact UTF-8
bytes, base64 signature, and base64 of the certificate file's exact bytes
with no parsing or re-encoding. Concurrent callers share one login via an
in-flight promise. Refresh is deliberately unimplemented: the collection
shows an unsigned refresh body but also ships unsigned examples of calls
that do require signing, so an expired token re-logs in instead.

Errors normalise to EimsApiException carrying only the gateway's own error
fields; secrets, signature, certificate and tokens never reach logs.
Key and certificate file patterns are gitignored.

Nothing calls EIMS automatically and no invoice entity, migration or UI is
touched.

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

apps/edr-freight-api/package.json

@@ -37,7 +37,8 @@
     "iam:migration:revert": "pnpm run iam:typeorm:cli migration:revert",
     "iam:migration:show": "pnpm run iam:typeorm:cli migration:show",
     "migration:run": "nest build && node dist/scripts/migrate.js",
-    "script": "ts-node -r tsconfig-paths/register src/scripts/main.ts"
+    "script": "ts-node -r tsconfig-paths/register src/scripts/main.ts",
+    "eims:login": "ts-node -r tsconfig-paths/register src/scripts/eims-login.ts"
   },
   "dependencies": {
     "@edr/api-common": "workspace:*",