feat(federation): grants service CRUD + status transitions (FED-M2-06)

- Add 'pending' to grantStatusEnum (pending → active → revoked/expired)
- Update federationGrants default status to 'pending'
- Add migration 0009_federation_grant_pending.sql
- GrantsService: createGrant, getGrant, listGrants, activateGrant, revokeGrant, expireGrant
- Invalid transitions throw ConflictException; missing grants throw NotFoundException
- CreateGrantDto validates scope via parseFederationScope before insert
- Full unit test coverage for all status transitions and edge cases

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

packages/db/drizzle/0009_federation_grant_pending.sql

@@ -0,0 +1,2 @@
+ALTER TYPE "public"."grant_status" ADD VALUE 'pending' BEFORE 'active';--> statement-breakpoint
+ALTER TABLE "federation_grants" ALTER COLUMN "status" SET DEFAULT 'pending';