feat(academy): learner progress tracking and certificates (BE-038)

[abiolatousin-design]

Overview

Adds per-learner progress tracking to BackendAcademy: lesson and task
completion counts and percentages, XP earned per course, an aggregate endpoint
for the dashboard, and the transition that marks a course complete — which is
what makes the learner eligible for a certificate.

The new ProgressService follows the academy's existing service pattern
(in-memory state, like XpService and FollowService) and reuses the
DomainEventBus / XpService that already exist in GamificationModule
instead of pricing XP itself: finishing a task publishes task.passed and
completing a course publishes course.completed, so XP stays in one place.
xpEarned per course is read back from that ledger by courseId.

Related Issue

Implements the per-learner progress tracking described in #791 (BE-038), under
the Learning Academy backlog area and the BackendAcademy scope.

Changes

• [ADD] BackendAcademy/src/progress/progress.types.ts

  • CourseContent (lesson/task manifest), CourseCompletion,
    CertificateEligibility, LearnerCourseProgress and LearnerDashboard.

• [ADD] BackendAcademy/src/progress/progress.service.ts

  • Registers a course's lesson and task manifest (rejects a manifest with no
    lessons or no tasks, since it could never be completed).
  • completeLesson() / completeTask() record completion and return refreshed
    course progress; a repeated completion is rejected with BadRequestException.
  • evaluateCompletion() flips the course to complete exactly once, when every
    lesson and task is done, then publishes course.completed and records a
    CertificateEligibility.
  • getCourseProgress() returns completion counts, rounded percentages and
    per-course xpEarned; getDashboard() aggregates courses, total XP and
    certificates for a learner; listCertificates() / getCertificate() expose
    certificate eligibility.

• [ADD] BackendAcademy/src/progress/progress.controller.ts

  • POST /api/v1/progress/courses — register a course manifest.
  • POST /api/v1/progress/lessons/complete — mark a lesson complete.
  • POST /api/v1/progress/tasks/complete — mark a task complete (awards XP).
  • GET /api/v1/progress/users/:userId/courses/:courseId — one course.
  • GET /api/v1/progress/users/:userId/dashboard — aggregate dashboard payload.
  • GET /api/v1/progress/users/:userId/certificates — certificate eligibility.

• [ADD] BackendAcademy/src/progress/dto/{register-course,complete-lesson,complete-task}.dto.ts

  • Validated request bodies, matching forbidNonWhitelisted global validation.

• [ADD] BackendAcademy/src/progress/progress.module.ts

  • Imports GamificationModule so completions publish on the shared event bus.

• [ADD] BackendAcademy/src/progress/progress.service.spec.ts

  • Unit tests for percentages, the completion → certificate transition,
    single-award idempotency, dashboard aggregation and error cases.

• [ADD] BackendAcademy/test/progress.e2e-spec.ts

  • Exercises the REST surface end to end through ProgressModule.

• [MODIFY] BackendAcademy/src/app.module.ts

  • Registers ProgressModule.

• [MODIFY] BackendAcademy/README.md

  • Lists the progress module.

Verification Results

No local checkout: the repository was inspected and changed through the GitHub
Contents/Git Data API, so `pnpm build` / `test` / `test:e2e` could NOT be run here.

What was verified:
✅ tsc --noEmit over the new and changed TypeScript files (dependencies mapped to
   local stubs, since node_modules is not present): 0 errors; the only
   diagnostics were the stubbed external module names.
✅ The assertions in progress.service.spec.ts were compiled and executed against
   a Node harness with test-double implementations of @nestjs/common and rxjs
   (jest is not installed here): 5 passed, 0 failed — covering lesson/task
   percentages, completion only after all lessons+tasks, certificate issued
   exactly once with the 110 XP total, cross-course dashboard aggregation, and
   the NotFound/Conflict/BadRequest paths.
⚠️ Not executed here: the real Jest unit run and the Supertest e2e run. A
   maintainer should run:
     pnpm --filter @rustacademy/backend-academy build
     pnpm --filter @rustacademy/backend-academy test
     pnpm --filter @rustacademy/backend-academy test:e2e

Acceptance Criteria	Status
Per-learner lesson completion %	✅ lessonCompletionPercent on LearnerCourseProgress
Task completion	✅ tasksCompleted / tasksTotal / taskCompletionPercent
XP earned per course	✅ xpEarned, read from the existing XP ledger by courseId
Aggregate endpoint for the dashboard	✅ GET /api/v1/progress/users/:userId/dashboard
Completing all lessons + tasks marks the course complete	✅ Single evaluateCompletion() transition sets completed / completedAt
Completion triggers certificate eligibility	✅ CertificateEligibility issued in the same transition and exposed via the API

Closes #791

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