feat: add public campaign impact summary with aggregate on-chain stats

[cythecode]

Overview

This PR adds a shareable Public Campaign Impact Summary feature that computes aggregate on-chain contribution statistics — total raised, contribution count, average contribution, largest contribution, unique contributor count, and currency — without exposing private contributor identities. Campaign creators and sponsors can cite verified fundraising totals to build trust, while contributors can see aggregate impact. The public endpoint is cached, rate-limited, and optionally signed by CrowdPay's platform key for independent provenance verification.

Related Issue

Closes #

Changes

📊 Aggregate Impact Reporting Engine

• [ADD] backend/src/services/impactReportService.js

  • Reads on-chain contribution records from escrow/indexer data.
  • Computes total_raised, contribution_count, average_contribution, largest_contribution, unique_contributor_count, and currency.
  • Recomputes correctly when new contributions or refunds occur.
  • Aggregates only — no contributor identities are ever included.

• [ADD] backend/src/services/impactSealService.js

  • Signs the aggregate stats payload with the CrowdPay platform key.
  • Optional signature lets third parties verify that the numbers originate from CrowdPay.

• [MODIFY] backend/src/routes/campaigns.js

  • Exposes GET /api/campaigns/:id/impact.
  • Returns aggregate stats only, plus the optional signed payload.
  • Enforces a minimum cache TTL of 60 seconds.

• [MODIFY] backend/src/routes/embed.js and backend/public/widget.js

  • Renders the impact badge on the public campaign page and in the embed widget.

• [MODIFY] backend/src/config/constants.js

  • Adds impact cache TTL, rate-limit thresholds, and signature configuration.

• [MODIFY] backend/src/middleware/rateLimiter.js

  • Rate-limits the public impact endpoint to prevent abuse.

• [ADD] backend/src/services/__tests__/impactReport.test.js

  • Covers aggregate correctness, PII leak prevention, caching behavior, and signature verification.

Verification Results

npm test -- backend/src/services/__tests__/impactReport.test.js
✅ 12/12 passed

Live acceptance check:
✅ Aggregates match on-chain escrow data after contributions/refunds
✅ No PII or individual contributor data exposed
✅ Cache TTL observed: minimum 60 seconds
✅ Public endpoint rate-limited (429 on burst)
✅ Impact badge renders in embed widget and public campaign page
✅ Signed payload verifies with CrowdPay platform key

Acceptance Criteria	Status
Public impact endpoint returns aggregate contribution stats	✅ Returns total_raised, contribution_count, average_contribution, largest_contribution, unique_contributor_count, currency
No PII/individual contributor data is exposed	✅ Aggregates only; contributor identities never included
Stats are cached for at least 60 seconds	✅ Minimum cache TTL enforced
Public endpoint is rate-limited	✅ Rate limiter returns 429 on abuse bursts
Impact badge renders on public campaign page and/or embed widget	✅ Badge rendered in both embed widget and campaign page
Optional signed payload lets third parties verify stats' provenance	✅ ImpactSealService signs payload; signature verifies with platform key
Stats recompute correctly when new contributions/refunds occur	✅ Recomputes from escrow/indexer data; tested with contribution and refund flows

Closes #767

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