feat(iios): generic interaction-annotation primitive (backs emoji reactions)

Adds IiosInteractionAnnotation — an actor attaches an OPAQUE (annotationType, value)
label to an interaction. The kernel stores + aggregates them but never interprets the
strings (the chat app writes type "reaction" / value = emoji); the same primitive backs
pins/saves/flags/tags later. No chat vocabulary in kernel code — verified by grep.

- MessageService.toggleAnnotation(): governed (participant-only via new OPA rule
  iios.interaction.annotate), idempotent toggle keyed on
  (scope, interaction, actor, type, value); history DTO carries aggregated
  { type, value, users[] } groups.
- Gateway: `annotate` event → broadcasts `annotation` (refreshed user list) to the room.
- DevOpaPort: annotate allowed only for thread members (real OPA can tighten later).
- Migration add_interaction_annotations (additive; dev data untouched).

Tests: 178 pass (+3: toggle/aggregate, coexisting values, non-member denied).

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

packages/iios-service/prisma/migrations/20260706182400_add_interaction_annotations/migration.sql

@@ -0,0 +1,27 @@
+-- CreateTable
+CREATE TABLE "IiosInteractionAnnotation" (
+    "id" TEXT NOT NULL,
+    "scopeId" TEXT NOT NULL,
+    "targetInteractionId" TEXT NOT NULL,
+    "actorId" TEXT NOT NULL,
+    "annotationType" TEXT NOT NULL,
+    "value" TEXT NOT NULL,
+    "createdAt" TIMESTAMP(3) NOT NULL DEFAULT CURRENT_TIMESTAMP,
+
+    CONSTRAINT "IiosInteractionAnnotation_pkey" PRIMARY KEY ("id")
+);
+
+-- CreateIndex
+CREATE INDEX "IiosInteractionAnnotation_targetInteractionId_idx" ON "IiosInteractionAnnotation"("targetInteractionId");
+
+-- CreateIndex
+CREATE UNIQUE INDEX "IiosInteractionAnnotation_scopeId_targetInteractionId_actor_key" ON "IiosInteractionAnnotation"("scopeId", "targetInteractionId", "actorId", "annotationType", "value");
+
+-- AddForeignKey
+ALTER TABLE "IiosInteractionAnnotation" ADD CONSTRAINT "IiosInteractionAnnotation_scopeId_fkey" FOREIGN KEY ("scopeId") REFERENCES "IiosScope"("id") ON DELETE CASCADE ON UPDATE CASCADE;
+
+-- AddForeignKey
+ALTER TABLE "IiosInteractionAnnotation" ADD CONSTRAINT "IiosInteractionAnnotation_targetInteractionId_fkey" FOREIGN KEY ("targetInteractionId") REFERENCES "IiosInteraction"("id") ON DELETE CASCADE ON UPDATE CASCADE;
+
+-- AddForeignKey
+ALTER TABLE "IiosInteractionAnnotation" ADD CONSTRAINT "IiosInteractionAnnotation_actorId_fkey" FOREIGN KEY ("actorId") REFERENCES "IiosActorRef"("id") ON DELETE RESTRICT ON UPDATE CASCADE;