feat: implement course progress persistence - #115

Closes #35

Implements database-backed course progress persistence. Previously, learning.service.ts used an in-memory Map that lost all progress on server restart. Now everything goes through Prisma → PostgreSQL.

What changed

New file: backend/src/learning/progress.ts

• getProgress(studentId) — retrieves persisted progress from PostgreSQL via Prisma, returns defaults for new students
• recordStageCompletion({ studentId, stageId, moduleId }, totalLessons) — idempotently records a completed stage, recalculates percentage
• resetProgress(studentId) — clears all progress for a student
• Flexible CompletedStage and StudentProgress interfaces designed for future module types (quizzes, projects, etc.)

Updated: backend/src/routes/learning/learning.service.ts

• Replaced in-memory Map storage with calls to the new progress.ts persistence module
• getStudentProgress and updateProgress now delegate to Prisma-backed functions

Updated: backend/src/routes/learning/learning.routes.ts

• Routes now use the service layer instead of inline Prisma calls
• Tracks which module a completed lesson belongs to
• Removed direct prisma import — all DB access goes through the service

New file: backend/tests/progress.test.ts

• Integration tests covering: default progress for new students, persisted progress retrieval, lesson completion, idempotency, percentage calculation, module tracking, validation errors, and data flexibility

Requirements from issue

• Create backend/src/learning/progress.ts
• Implement a function to record completed stages for a student ID
• Ensure the data structure is flexible for future modules (CompletedStage interface with stageId, moduleId, completedAt)

Data structure

// Flexible stage tracking — works for lessons, quizzes, projects, etc.
interface CompletedStage {
  stageId: string;
  completedAt: string;   // ISO 8601
  moduleId: string;
}

interface StudentProgress {
  userId: string;
  completedLessons: string[];
  completedStages: CompletedStage[];
  currentModule: string;
  percentage: number;
  lastActivityAt: string;
}

How to test

cd backend
npm test -- --testPathPattern=progress