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/src/modules/classes/lib/helpers.ts

@@ -0,0 +1,92 @@
+import type { PrismaClient } from '@generated/prisma/client';
+import type {
+  AttendeePaymentStatus,
+  ClassToday,
+  ProblemDetails,
+  Result,
+} from '@gruperly/shared';
+import { err } from '@gruperly/shared';
+import { noGroupAccessProblem, notFoundResourceProblem } from '@/http/problem-builders';
+
+export type ClassesDb = Pick<
+  PrismaClient,
+  'group' | 'classSession' | 'attendee' | 'payment' | 'attendance' | 'slotRelease'
+>;
+
+type PaymentLike = {
+  status: 'PENDING' | 'PAID' | 'OVERDUE' | 'CANCELLED';
+  dueDate: Date;
+};
+
+// Estado de cobro calculado en tiempo real (sin columna denormalizada):
+// PENDIENTE si hay un pago vencido (OVERDUE) o un PENDING cuyo vencimiento ya pasó.
+export function resolvePaymentStatus(
+  payments: PaymentLike[],
+  startOfToday: Date,
+): AttendeePaymentStatus {
+  const hasUnpaid = payments.some(
+    (payment) =>
+      payment.status === 'OVERDUE'
+      || (payment.status === 'PENDING' && payment.dueDate.getTime() < startOfToday.getTime()),
+  );
+  return hasUnpaid ? 'PENDING' : 'UP_TO_DATE';
+}
+
+export function absenceReleaseHours(): number {
+  const raw = process.env.ABSENCE_RELEASE_HOURS;
+  if (raw == null || raw.trim() === '') {
+    return 12;
+  }
+  const parsed = Number(raw);
+  return Number.isFinite(parsed) && parsed >= 0 ? parsed : 12;
+}
+
+export type ClassSessionRecord = {
+  id: string;
+  groupId: string;
+  startsAt: Date;
+};
+
+type GroupWithAccess = {
+  createdById: string;
+  members: { userId: string }[];
+};
+
+// Verifica que el usuario sea owner o miembro del grupo dueño de la sesión.
+export function ensureSessionAccess(
+  session: ClassSessionRecord & { group: GroupWithAccess },
+  userId: string,
+): Result<ClassSessionRecord, ProblemDetails> {
+  const isOwner = session.group.createdById === userId;
+  const isMember = session.group.members.length > 0;
+  if (!isOwner && !isMember) {
+    return err(noGroupAccessProblem());
+  }
+  return {
+    ok: true,
+    value: { id: session.id, groupId: session.groupId, startsAt: session.startsAt },
+  };
+}
+
+export function sessionNotFound(sessionId: string): ProblemDetails {
+  return notFoundResourceProblem('Class session', sessionId);
+}
+
+export function toClassToday(
+  session: { id: string; startsAt: Date },
+  group: { id: string; name: string; capacity: number | null },
+  enrolledCount: number,
+  hasAttendance: boolean,
+): ClassToday {
+  return {
+    sessionId: session.id,
+    groupId: group.id,
+    groupName: group.name,
+    startsAt: session.startsAt.toISOString(),
+    enrolledCount,
+    capacity: group.capacity,
+    availableSlots:
+      group.capacity == null ? null : Math.max(0, group.capacity - enrolledCount),
+    hasAttendance,
+  };
+}