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/src/modules/eims/eims-credentials.provider.ts

@@ -0,0 +1,77 @@
+import { readFileSync } from "node:fs";
+import { KeyObject, createPrivateKey } from "node:crypto";
+import { Injectable, Logger } from "@nestjs/common";
+import { ConfigService } from "@nestjs/config";
+import { EimsConfig } from "../../config/eims.config";
+import { EimsConfigException } from "./eims.errors";
+
+/**
+ * Loads the INSA-issued EIMS credentials from disk, once, and keeps them in memory.
+ *
+ * The certificate is sent as base64 of the **exact bytes of the issued file** — it is deliberately
+ * never parsed, re-encoded or re-exported, because that is what produced a working live login.
+ * The private key never leaves this process: it is only ever used to produce a signature.
+ */
+@Injectable()
+export class EimsCredentialsProvider {
+  private readonly logger = new Logger(EimsCredentialsProvider.name);
+  private privateKey: KeyObject | null = null;
+  private certificateBase64: string | null = null;
+
+  constructor(private readonly config: ConfigService) {}
+
+  private get cfg(): EimsConfig {
+    return this.config.get<EimsConfig>("eims")!;
+  }
+
+  /** RSA private key, parsed once. Throws a config error if the path is missing or unusable. */
+  getPrivateKey(): KeyObject {
+    if (this.privateKey) return this.privateKey;
+
+    const path = this.cfg.privateKeyPath;
+    if (!path) throw new EimsConfigException("EIMS_PRIVATE_KEY_PATH is not set");
+
+    let key: KeyObject;
+    try {
+      key = createPrivateKey(readFileSync(path));
+    } catch (err) {
+      // The path is operational information, not a secret; the key material never appears.
+      throw new EimsConfigException(
+        `EIMS private key at ${path} could not be read or parsed: ${(err as Error).message}`,
+      );
+    }
+    if (key.asymmetricKeyType !== "rsa") {
+      throw new EimsConfigException(
+        `EIMS private key at ${path} is ${key.asymmetricKeyType ?? "of unknown type"}; EIMS requires RSA`,
+      );
+    }
+
+    this.privateKey = key;
+    this.logger.log(`EIMS private key loaded (RSA-${key.asymmetricKeyDetails?.modulusLength ?? "?"})`);
+    return key;
+  }
+
+  /** Base64 of the certificate file's exact bytes. No parsing, no re-encoding. */
+  getCertificateBase64(): string {
+    if (this.certificateBase64) return this.certificateBase64;
+
+    const path = this.cfg.certificatePath;
+    if (!path) throw new EimsConfigException("EIMS_CERTIFICATE_PATH is not set");
+
+    let bytes: Buffer;
+    try {
+      bytes = readFileSync(path);
+    } catch (err) {
+      throw new EimsConfigException(
+        `EIMS certificate at ${path} could not be read: ${(err as Error).message}`,
+      );
+    }
+    if (bytes.length === 0) {
+      throw new EimsConfigException(`EIMS certificate at ${path} is empty`);
+    }
+
+    this.certificateBase64 = bytes.toString("base64");
+    this.logger.log(`EIMS certificate bundle loaded (${bytes.length} bytes)`);
+    return this.certificateBase64;
+  }
+}