feat(db): add good_origin_goods junction table for merged origin goods

apps/api/prisma/schema.prisma

@@ -36,6 +36,7 @@ model OriginGood {
   goods    Good[]
   detail   OriginGoodDetail?
   variants OriginGoodVariant[]
+  mergedIntoGoods GoodOriginGood[]
 
   @@index([sdsCategoryId])
   @@index([source])
@@ -215,6 +216,7 @@ model Good {
   tag        Tag?       @relation(fields: [tagId], references: [id], onDelete: SetNull, onUpdate: NoAction)
   position   Position?  @relation(fields: [positionId], references: [id], onDelete: SetNull, onUpdate: NoAction)
   goodTags   GoodTag[]
+  mergedOriginGoods GoodOriginGood[]
 
   @@index([originGoodId])
   @@index([countryId])
@@ -241,6 +243,21 @@ model GoodTag {
   @@map("good_tags")
 }
 
+// ---------- Good-OriginGood Junction (merged secondary sources, M:N) ----------
+// Primary source stays on goods.origin_good_id and is NOT stored here.
+model GoodOriginGood {
+  goodId       BigInt   @map("good_id")
+  originGoodId BigInt   @map("origin_good_id")
+  createdAt    DateTime @default(now()) @map("created_at") @db.Timestamptz(6)
+
+  good       Good       @relation(fields: [goodId], references: [id], onDelete: Cascade, onUpdate: NoAction)
+  originGood OriginGood @relation(fields: [originGoodId], references: [id], onDelete: Cascade, onUpdate: NoAction)
+
+  @@id([goodId, originGoodId])
+  @@index([originGoodId])
+  @@map("good_origin_goods")
+}
+
 // ---------- Users (admin authentication) ----------
 enum Role {
   ADMIN