MTL-127: Add PDF report download

[LeonSilva15]

Summary:

• Add a ReportLab-backed PDF report download endpoint generated from the persisted EPD-style report DTO.
• Add report page PDF download UI with loading, success, and error feedback.
• Document PDF as an optional presentation export, not a verified EPD.
• Preserve report context snapshots so historical JSON/PDF exports do not drift when current assessment, product, plant, manufacturer, inputs, factors, or findings change.

Validation:

• uv run --project backend pytest backend/app/tests/test_assessment_report_api.py backend/app/tests/test_report_export_service.py backend/app/tests/test_calculation_run_persistence.py: passed, 28 tests
• uv run --project backend pytest: passed, 312 tests
• uv run --project backend ruff check .: passed
• uv run --project backend ruff format backend/app/domain/calculation.py backend/app/domain/calculation_runs.py backend/app/domain/report_export.py backend/app/domain/report_pdf.py backend/app/tests/test_assessment_report_api.py backend/app/tests/test_calculation_run_persistence.py: passed, unchanged
• uv run --project backend mypy app: passed
• npm --prefix frontend run lint: passed
• npm --prefix frontend run format:check: passed
• npm --prefix frontend run build: passed
• git diff --check: passed
• git diff --check upstream/master..HEAD: passed

Review:

• One subagent review completed.
• Fixed accepted findings: historical report context drift and PDF determinism wording/byte stability.

Evidence:

Playwright Evidence

Committed evidence directory: skills/materialtrace-demo-evidence/MTL-127

pdf-report-download (unknown)

• Run directory: skills/materialtrace-demo-evidence/MTL-127/20260527T032120Z-pdf-report-download
• Artifacts: Summary | Payload Examples | Trace | Requests | Responses | Console | Errors | Manifest
• Screenshot: MTL-127 pdf-report-download screenshot 1
• Screenshot: MTL-127 pdf-report-download screenshot 2

MaterialTrace ticket: MTL-127