feat(reports): add pdf report export

[Nekolandd]

Description

Adds PDF export for security findings reports at src/modules/reports/pdf/, allowing stakeholders to download executive security summaries as PDF files.

Implements a new ReportsModule with a download endpoint that reuses existing ReportingService data and renders it through a dedicated PDF template using pdfkit.

Type of Change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Documentation update

Related Issues

Fixes #129

Changes Made

• PDF generation — src/modules/reports/pdf/pdf-report.service.ts using pdfkit
• Report template — src/modules/reports/pdf/pdf-report.template.ts (executive summary, severity breakdown, top affected chains)
• Download endpoint — GET /api/reports/security/pdf (optional ?days=7) returns downloadable PDF via StreamableFile
• Module wiring — src/modules/reports/reports.module.ts registered in apps/backend/src/app.module.ts
• Build config — apps/backend/tsconfig.app.json includes src/modules/reports for backend compilation
• Dependency — added pdfkit + @types/pdfkit
• Tests — 6 unit tests (service, template, controller)

Testing

• I have tested these changes locally
• Tests pass locally — 6/6 (npm run test:backend -- --testPathPatterns=reports)
• Linting passes on reports files (eslint + prettier --write)
• Code is formatted (prettier --write on reports files)
• TypeScript builds successfully (npm run build)

How to verify:

• npm run prisma:generate
• npm run build
• npm run test:backend -- --testPathPatterns=reports

Manual download test (with backend running):

• curl -o report.pdf "http://localhost:3000/api/reports/security/pdf?days=7"

Checklist

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings or errors
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests passed with my changes

Additional Context

• Implementation scope: follows issue path src/modules/reports/pdf/ as specified in Add PDF Report Export #129.
• Data source: reuses ReportingService.getSecurityReport() from the existing reporting module in apps/backend/src/modules/reporting/.
• Cross-module imports: src/modules/reports imports reporting types/services from apps/backend/; tsconfig.app.json was updated to compile the new module.
• New dependency: pdfkit is used for server-side PDF generation.
• Acceptance criteria met:
  • PDF export working
  • Templates implemented
  • Reports downloadable