feat(reports): add instructor performance report API (#65) - #191

Closes #65

Problem Statement (The Bug)

The platform has no instructor-performance reporting. Course carries
denormalised counters (avgRating, totalRevenue, totalEnrollments) but:

1. they are per-course, not per-instructor — there is no query that rolls
   an instructor's whole catalogue into one view;
2. they are all-time — they cannot answer "how did this instructor do in
   Q1?";
3. there is no benchmark — a raw avgRating of 4.2 is meaningless
   without the platform average next to it.

This can't be a local patch because the missing capability is a
cross-entity, period-bounded read model (courses × enrollments × reviews ×
platform aggregates). Reading the denormalised Course columns would bake in
both the all-time and drift problems (those counters are updated by other
code paths and can fall out of sync).

Solution Comparison and Decision

The Change

New DTO — InstructorReportQueryDto (from?, to?, both ISO-8601).

New core method — InstructorReportService.getReport(instructorId, dto):

const instructor = await this.prisma.user.findUnique({ where: { id: instructorId }, select: { id, name, role, stellarAddress } });
if (!instructor) throw new NotFoundException('Instructor not found');
if (instructor.role !== 'INSTRUCTOR') throw new BadRequestException('User is not an instructor');

Revenue is computed per enrollment against its course's own
platformFeePercent, not a flat rate:

const platformFees = enrollments.reduce((sum, e) => {
  const feePercent = feeByCourse.get(e.courseId) ?? 0;
  return sum + (this.toNumber(e.amountPaid) * feePercent) / 100;
}, 0);

compare() turns each metric into an instructor-vs-platform point:

private compare(instructor: number, platformAverage: number): ComparisonPoint {
  const delta = this.round(instructor - platformAverage);
  const percentDiff = platformAverage === 0 ? null : this.round((delta / platformAverage) * 100);
  const verdict = delta > 0 ? 'above' : delta < 0 ? 'below' : 'on_par';
  return { instructor, platformAverage, delta, percentDiff, verdict };
}

Sample response:

{
  "instructor": { "id": "inst-1", "name": "Ada", "stellarAddress": "GABC" },
  "period": { "from": null, "to": null },
  "generatedAt": "2026-08-27T12:00:00.000Z",
  "metrics": {
    "courses": { "total": 2, "active": 1 },
    "ratings": { "average": 4.5, "totalReviews": 8 },
    "students": { "enrollments": 3, "completions": 2, "completionRate": 0.6667 },
    "revenue": { "gross": 250, "platformFees": 45, "net": 205 }
  },
  "platformAverages": {
    "instructorCount": 4, "rating": 4, "completionRate": 0.5,
    "grossRevenuePerInstructor": 1000, "enrollmentsPerInstructor": 20
  },
  "comparison": {
    "rating":         { "instructor": 4.5, "platformAverage": 4,    "delta": 0.5,   "percentDiff": 12.5, "verdict": "above" },
    "completionRate": { "instructor": 0.6667, "platformAverage": 0.5, "delta": 0.17, "percentDiff": 33.4, "verdict": "above" },
    "grossRevenue":   { "instructor": 250, "platformAverage": 1000,  "delta": -750,  "percentDiff": -75,  "verdict": "below" },
    "enrollments":    { "instructor": 3,   "platformAverage": 20,    "delta": -17,   "percentDiff": -85,  "verdict": "below" }
  }
}

Compatibility Note

No INTERFACE_VERSION constant exists in this repo. The change is
additive: one new route, one new DTO, one new service.
ReportsModule gains a controller/provider; the existing abuse-report
ReportsController / ReportsService and the /reports routes are
unchanged. No schema change, no migration.

Incidental Fixes

• ReportsModule now exports its providers so InstructorReportService
  can be consumed elsewhere (same pattern the abuse ReportsService used).
• Divide-by-zero guards throughout (rate(), per-instructor divisor
  instructorCount || 1, percentDiff null when the platform average is 0)
  so an instructor with no courses / an empty platform returns 0 / null,
  never NaN.

Testing

src/modules/reports/instructor-report.service.spec.ts — resolves the real
InstructorReportController + InstructorReportService from a Nest module
(Prisma mocked) and calls through controller.getReport(...):

• 404s for an unknown instructor
• 400s when the user is not an instructor — adversarial role check
• rejects an inverted reporting period — adversarial from > to
• passes the reporting period through to the enrollment query
• computes per-instructor rating, revenue (with per-course fees) and completion rate — asserts gross 250 / fees 45 (20% + 20% + 10%) / net 205,
  completion 0.6667
• compares the instructor against platform averages — delta / verdict for
  rating (above) and gross revenue (below)
• handles an instructor with no courses without dividing by zero

$ npx jest src/modules/reports/instructor-report.service.spec.ts
Tests:       7 passed, 7 total

Pre-existing, unrelated TypeScript failures in src/modules/reviews/* and
src/modules/uploads/video-transcode.service.ts exist on main and are not
touched here.

Additional Notes

• No base-branch fixup commits.
• Scope: src/modules/reports/ only — one DTO, one controller, one
  service, module wiring. No change to the abuse-report path, to
  prisma/schema.prisma, or to any other module. Student-engagement
  reporting ([Backend] Create student engagement report API #64) is a separate PR.