feat(testing): add N+1 query detection helper and fix list endpoints (#1243) - #1423

Overview

Implements automated N+1 query detection and regression testing helpers for Prisma, and resolves existing N+1 query patterns across profiles and analytics services.

Related Issue

Closes #1243

Changes

Testing Framework & Database Observability

• [ADD] backend/src/common/testing/queryCounter.ts
  • Added queryCounterMiddleware using Node's AsyncLocalStorage to track Prisma queries per async context.
  • Added countQueries(action) helper to inspect exact queries and execution count.
  • Added assertConstantQueryCount(runner, sizes) helper to verify that query count remains constant regardless of result-set / page size.
• [ADD] backend/src/common/testing/queryCounter.test.ts
  • 10 unit tests verifying middleware passthrough, context isolation across concurrent calls, duration tracking, and failure diagnostics.
• [ADD] backend/tests/helpers/queryCounter.ts
  • Re-exported testing helpers for integration tests.
• [ADD] backend/tests/nPlusOne.test.ts
  • 17 tests verifying N+1 prevention across profiles, analytics, leaderboard, tips, notifications, goals, webhooks, subscriptions, and discovery services.
• [MODIFY] backend/src/db/prisma.ts
  • Registered queryCounterMiddleware on the singleton Prisma client (no-op in production, active only within test counting contexts).

Service Optimization & N+1 Fixes

• [MODIFY] backend/src/modules/profiles/profiles.service.ts
  • Replaced per-profile getTipStats() calls in listProfiles with a single batched prisma.tip.groupBy({ where: { toAddress: { in: addresses } } }) query, reducing queries from O(2N + 1) to O(3) constant.
• [MODIFY] backend/src/modules/analytics/analytics.service.ts
  • Replaced per-row prisma.user.findUnique lookups in getTopTippers with a single batched prisma.user.findMany({ where: { stellarAddress: { in: addresses } } }).
  • Replaced per-row prisma.user.findUnique lookups for top tippers in getCreatorAnalytics with a single batched prisma.user.findMany.

Documentation

• [ADD] backend/docs/N_PLUS_ONE_DETECTION.md
  • Complete documentation on N+1 query detection, usage examples, context architecture, and guidelines for testing new list endpoints.
• [MODIFY] backend/docs/BACKEND_CONTRIBUTING.md
  • Added N+1 query guard guidelines and test requirements for all list/pagination endpoints.

Verification Results

✓ src/common/testing/queryCounter.test.ts (10 tests)
✓ tests/nPlusOne.test.ts (17 tests)

Test Files  2 passed (2)
     Tests  27 passed (27)