fix(lms): support direct R2 map uploads and preserve empty course groups

prisma/schema.prisma

@@ -65,6 +65,8 @@ model StudentEnrollment {
 
 model StudentGroup {
   id               String               @id @default(uuid()) @db.Uuid
+  courseId         String?              @map("course_id") @db.Uuid
+  course           Course?              @relation(fields: [courseId], references: [id], onDelete: Restrict)
   organizationId   String?              @map("organization_id") @db.Uuid
   organizationName String               @map("organization_name")
   name             String
@@ -84,6 +86,7 @@ model StudentGroup {
   teamCustomModules TeamCustomModule[]
 
   @@unique([organizationName, name])
+  @@index([courseId])
   @@index([organizationId])
   @@index([organizationName])
   @@index([instructorPlatformId])
@@ -168,6 +171,7 @@ model Course {
   customModules   StudentCustomModule[]
   teamCustomModules TeamCustomModule[]
   submissionScores SubmissionScore[]
+  studentGroups    StudentGroup[]
   submissionTotalScores SubmissionTotalScore[]
 
   @@index([isActive])