feat(gif-mood): hide/reveal GIFs from other participants

- Add hidden field to GifMoodUserRating (schema + migration)
- Add setGifMoodUserHidden service + action with SSE broadcast
- Current user sees Cacher/Révéler toggle in their section header
- Other users see a locked placeholder with item count when hidden

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

prisma/schema.prisma

@@ -561,7 +561,8 @@ model GifMoodUserRating {
   session   GifMoodSession @relation(fields: [sessionId], references: [id], onDelete: Cascade)
   userId    String
   user      User           @relation(fields: [userId], references: [id], onDelete: Cascade)
-  rating    Int            // 1-5
+  rating    Int?           // 1-5
+  hidden    Boolean        @default(false)
   createdAt DateTime       @default(now())
   updatedAt DateTime       @updatedAt