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/schema.prisma

@@ -303,6 +303,7 @@ model IiosScope {
   channels      IiosChannel[]
   threads       IiosThread[]
   interactions  IiosInteraction[]
+  annotations   IiosInteractionAnnotation[]
   inboxItems    IiosInboxItem[]
   supportQueues IiosSupportQueue[]
   tickets       IiosTicket[]
@@ -348,6 +349,7 @@ model IiosActorRef {
   threadsCreated     IiosThread[]            @relation("ThreadCreatedBy")
   participations     IiosThreadParticipant[]
   interactions       IiosInteraction[]
+  annotations        IiosInteractionAnnotation[]
   receipts           IiosMessageReceipt[]
   unreadCounters     IiosUnreadCounter[]
   inboxItemsOwned    IiosInboxItem[]
@@ -444,6 +446,7 @@ model IiosInteraction {
 
   parts             IiosMessagePart[]
   receipts          IiosMessageReceipt[]
+  annotations       IiosInteractionAnnotation[]
   inboxItems        IiosInboxItem[]
   ticketsCreatedFrom IiosTicket[]
 
@@ -451,6 +454,26 @@ model IiosInteraction {
   @@index([threadId, occurredAt])
 }
 
+/// A generic annotation an actor attaches to an interaction. Both `annotationType`
+/// and `value` are OPAQUE, app-supplied strings the kernel stores and aggregates
+/// but never interprets — e.g. the chat app writes type "reaction" / value "👍".
+/// The same primitive backs pins, saves, flags, tags later. No chat vocabulary here.
+model IiosInteractionAnnotation {
+  id                  String          @id @default(cuid())
+  scopeId             String
+  scope               IiosScope       @relation(fields: [scopeId], references: [id], onDelete: Cascade)
+  targetInteractionId String
+  target              IiosInteraction @relation(fields: [targetInteractionId], references: [id], onDelete: Cascade)
+  actorId             String
+  actor               IiosActorRef    @relation(fields: [actorId], references: [id])
+  annotationType      String
+  value               String
+  createdAt           DateTime        @default(now())
+
+  @@unique([scopeId, targetInteractionId, actorId, annotationType, value])
+  @@index([targetInteractionId])
+}
+
 model IiosMessagePart {
   id             String              @id @default(cuid())
   interactionId  String