feat: revocable read-only API keys for machine access

Integrations such as the Wabenhain shop sync no longer need a service
account with a password (and without MFA). An admin creates a key under
Verwaltung > Automatisierung > API-Schlüssel.

- Key format ack_<8>_<43>; only its SHA-256 hash is stored, the key is
  shown exactly once. Optional expiry, "last used" timestamp, revocation
  takes effect immediately; create/revoke are audit-logged without the key.
- A key is not a user: JwtAuthGuard sets request.apiKey, PermissionsGuard
  admits it only to routes whose permission is among its scopes and never
  to user-only routes. Scopes come from a short allowlist (shop-sync:read).
- Managing keys requires the new permission api-keys:manage.
- Keys are excluded from backups on purpose; a restore means issuing a new
  key.

Additive migration (new table).

Co-Authored-By: Claude Opus 5.5 (1M context) <[email protected]>

ApiCore-Backend/src/api-keys/api-keys.module.ts

@@ -0,0 +1,12 @@
+import { Global, Module } from '@nestjs/common';
+import { ApiKeysController } from './api-keys.controller';
+import { ApiKeysService } from './api-keys.service';
+
+// Global: the app-wide auth guards need ApiKeysService to recognise keys.
+@Global()
+@Module({
+  controllers: [ApiKeysController],
+  providers: [ApiKeysService],
+  exports: [ApiKeysService],
+})
+export class ApiKeysModule {}