feat(activities): let a group show and limit its members

Empty membership stays open to the whole workspace; picking people restricts who can see that activity group.

Co-authored-by: Cursor <cursoragent@cursor.com>

apps/api/prisma/schema.prisma

@@ -311,6 +311,7 @@ model User {
   tablePreferences       UserTablePreference[]
   assignedProjectCards   ProjectCard[]            @relation("ProjectCardAssignees")
   teamMemberships        TeamMember[]
+  activityBoardMembers   ActivityBoardMember[]
   reservationAssignments ReservationAppointment[] @relation("ReservationAssignee")
   assignedLeads          Lead[]                   @relation("LeadAssignee")
   createdLeads           Lead[]                   @relation("LeadCreator")
@@ -766,6 +767,7 @@ model ActivityBoard {
   position       Int             @default(0)
   stages         ActivityStage[]
   activities     Activity[]
+  members        ActivityBoardMember[]
   createdAt      DateTime        @default(now())
   updatedAt      DateTime        @updatedAt
 
@@ -773,6 +775,17 @@ model ActivityBoard {
   @@index([organizationId, position])
 }
 
+/// عضو گروه فعالیت؛ اگر گروهی هیچ عضوی نداشته باشد برای همهٔ محیط باز است
+model ActivityBoardMember {
+  boardId String
+  board   ActivityBoard @relation(fields: [boardId], references: [id], onDelete: Cascade)
+  userId  String
+  user    User          @relation(fields: [userId], references: [id], onDelete: Cascade)
+
+  @@id([boardId, userId])
+  @@index([userId])
+}
+
 model ActivityStage {
   id             String         @id @default(cuid())
   organizationId String