feat(#462): add federation capabilities verb (#674)

apps/gateway/src/federation/server/verbs/capabilities.controller.ts

@@ -0,0 +1,38 @@
+/**
+ * Federation capabilities verb (FED-M3-07).
+ *
+ * Returns the read-only capability envelope for the active grant attached by
+ * FederationAuthGuard. This endpoint intentionally does not invoke native RBAC
+ * or ScopeService: an active grant is enough to ask what the grant allows.
+ */
+
+import { Controller, Get, Req, UseGuards } from '@nestjs/common';
+import type { FastifyRequest } from 'fastify';
+import {
+  FEDERATION_VERBS,
+  type FederationCapabilitiesResponse,
+  type FederationVerb,
+} from '@mosaicstack/types';
+import { parseFederationScope } from '../../scope-schema.js';
+import { FederationAuthGuard } from '../federation-auth.guard.js';
+import '../federation-context.js';
+
+@Controller('api/federation/v1/capabilities')
+@UseGuards(FederationAuthGuard)
+export class CapabilitiesController {
+  @Get()
+  getCapabilities(@Req() request: FastifyRequest): FederationCapabilitiesResponse {
+    if (!request.federationContext) {
+      throw new Error('Federation context missing after auth guard');
+    }
+
+    const scope = parseFederationScope(request.federationContext.scope);
+
+    return {
+      resources: [...scope.resources],
+      excluded_resources: [...scope.excluded_resources],
+      max_rows_per_query: scope.max_rows_per_query,
+      supported_verbs: [...FEDERATION_VERBS] satisfies FederationVerb[],
+    };
+  }
+}