feat: campaign analytics report export to pdf

[Stanley-Owoh]

feat: Campaign analytics report export to PDF

Overview

Adds campaign creator analytics export to PDF, complementing the existing CSV export (#408). Creators can now generate a polished, printable financial and engagement report for their campaign, and share it via an expiring signed URL.

Changes

Backend

New: backend/src/services/campaignReportService.js

• Assembles all report data in a single payload via parallel DB queries: campaign summary, financials (raised vs goal, platform fees, net received, average/largest contribution), engagement (total contributions, unique contributors, asset breakdown), top contributors (wallet keys truncated for privacy), milestone status, and status-change timeline.
• Signed URL generation/verification: HMAC-signed, expiring (24h), and bound to the campaign id. Verification uses constant-time comparison with explicit length checks.

New: backend/src/services/campaignReportPdf.js

• Renders the report to PDF with pdfkit, including: campaign summary, raised vs goal (with progress bar), fee breakdown, contributor count, top contributors, milestone status, and funding/status timeline.
• streamCampaignReportPdf pipes PDF bytes directly to the HTTP response (no large in-memory buffer) as required by the issue.
• Handles an empty campaign gracefully (no contributors / no milestones → placeholder text).
• generateCampaignReportPdfBuffer provided for tests / object-storage flows.

backend/src/routes/campaigns.js — new routes:

• GET /api/campaigns/:id/report/export — streams a PDF attachment; requires authentication and owner (or admin) membership.
• GET /api/campaigns/:id/report/share — returns an expiring signed share URL; requires authentication and owner (or admin) membership.
• GET /api/campaigns/:id/report/share/:token — serves the PDF via a validated signed token (shareable link, no auth required, token-expiry enforced).

backend/package.json — added pdfkit dependency.

Frontend

• frontend/src/pages/CreatorCampaignAnalytics.jsx — added a "Download Report" button beside the existing CSV export, with loading state.
• frontend/src/services/api.js — added exportCampaignReport() blob download helper.

Tests

New: backend/src/services/campaignReportService.test.js

• Report assembly (financials, milestones, top contributors)
• Empty campaign handled gracefully
• Signed URL generation and validation
• Expired token rejection
• Tampered signature rejection
• Wrong-campaign token rejection

backend/src/routes/campaigns.test.js — added route coverage:

• Owner streams PDF (200, correct headers/body)
• Non-owner rejected (403)
• Auth required (401)
• Signed URL generation endpoint
• Valid signed token serves PDF
• Invalid/expired token rejected (403)

All backend tests pass (39 tests).

Closes #765

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