feat(activities): let creators lock edit and delete to themselves

Add editPolicy (default creator-only), enforce it in the API, surface clear permission messages, and drop the redundant edit menu now that detail is editable.

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

apps/api/prisma/migrations/20261111160000_activity_edit_policy/migration.sql

@@ -0,0 +1,5 @@
+-- CreateEnum
+CREATE TYPE "ActivityEditPolicy" AS ENUM ('creator', 'assignees');
+
+-- AlterTable
+ALTER TABLE "Activity" ADD COLUMN "editPolicy" "ActivityEditPolicy" NOT NULL DEFAULT 'creator';

apps/api/prisma/schema.prisma

@@ -38,6 +38,12 @@ enum ActivityCompletionMode {
   all
 }
 
+/// چه کسانی می‌توانند فعالیت را ویرایش/حذف کنند (علاوه بر owner/admin)
+enum ActivityEditPolicy {
+  creator
+  assignees
+}
+
 enum ActivityPriority {
   low
   medium
@@ -892,6 +898,8 @@ model Activity {
   completionMode    ActivityCompletionMode @default(any)
   /// مسئولانی که سهم خود را در حالت all ثبت کرده‌اند
   completedByIds    String[]         @default([])
+  /// creator = فقط ایجادکننده؛ assignees = ایجادکننده + همهٔ مسئولان
+  editPolicy        ActivityEditPolicy @default(creator)
   tags              String[]         @default([])
   personId          String?
   person            Person?          @relation(fields: [personId], references: [id], onDelete: SetNull)