fix(boards): keep card order inside a column and show activity priority

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

apps/api/prisma/migrations/20261103210000_activity_board_position/migration.sql

@@ -0,0 +1,3 @@
+ALTER TABLE "Activity" ADD COLUMN "boardPosition" INTEGER NOT NULL DEFAULT 0;
+
+CREATE INDEX "Activity_stageId_boardPosition_idx" ON "Activity"("stageId", "boardPosition");

apps/api/prisma/schema.prisma

@@ -896,6 +896,8 @@ model Activity {
   /// مرحله (ستون کانبان)؛ null یا مرحلهٔ ناهم‌نوع با status → اولین مرحلهٔ هم‌نوع
   stageId           String?
   stage             ActivityStage?   @relation(fields: [stageId], references: [id], onDelete: SetNull)
+  /// ترتیب کارت داخل همان ستون کانبان
+  boardPosition     Int              @default(0)
   /// نوع سفارشی؛ null → نوع اصلیِ همان type
   typeDefId         String?
   typeDef           ActivityTypeDef? @relation(fields: [typeDefId], references: [id], onDelete: SetNull)
@@ -911,6 +913,7 @@ model Activity {
   updatedAt         DateTime         @updatedAt
 
   @@index([stageId])
+  @@index([stageId, boardPosition])
   @@index([boardId])
   @@index([typeDefId])
   @@index([leadId])