feat: add absence notification and analytics features

- Implemented AbsenceNotifyView for notifying absence with session details.
- Created AnalyticsView for displaying group analytics and managing students at risk.
- Added ClassAttendanceView for marking attendance in classes.
- Defined new schemas for analytics and attendance in shared package.

apps/backend/prisma/models/attendance.prisma

@@ -0,0 +1,50 @@
+// Attendance & class sessions
+
+enum AttendanceStatus {
+  PRESENT
+  ABSENT
+  EXCUSED
+}
+
+model ClassSession {
+  id        String   @id @default(cuid())
+  groupId   String
+  startsAt  DateTime
+  createdAt DateTime @default(now())
+  updatedAt DateTime @updatedAt
+
+  group        Group         @relation(fields: [groupId], references: [id], onDelete: Cascade)
+  attendances  Attendance[]
+  slotReleases SlotRelease[]
+
+  @@unique([groupId, startsAt])
+  @@map("class_sessions")
+}
+
+model Attendance {
+  id             String           @id @default(cuid())
+  attendeeId     String
+  classSessionId String
+  status         AttendanceStatus
+  markedAt       DateTime         @default(now())
+
+  attendee     Attendee     @relation(fields: [attendeeId], references: [id], onDelete: Cascade)
+  classSession ClassSession @relation(fields: [classSessionId], references: [id], onDelete: Cascade)
+
+  @@unique([attendeeId, classSessionId])
+  @@map("attendances")
+}
+
+model SlotRelease {
+  id             String   @id @default(cuid())
+  attendeeId     String
+  classSessionId String
+  isClaimed      Boolean  @default(false)
+  createdAt      DateTime @default(now())
+
+  attendee     Attendee     @relation(fields: [attendeeId], references: [id], onDelete: Cascade)
+  classSession ClassSession @relation(fields: [classSessionId], references: [id], onDelete: Cascade)
+
+  @@unique([attendeeId, classSessionId])
+  @@map("slot_releases")
+}