feat(analytics): add cohort retention and churn HTTP endpoints

[retkatmun]

Closes #1755

Summary

Churn was not measured and cohort retention views did not exist over HTTP. The underlying CohortService and AnalyticsDashboardApi already contained the full cohort/churn/retention logic — this change wires it to the HTTP layer and adds a test suite.

Changes

New files:

File	Purpose
backend/analytics/cohortAnalyticsRouter.ts	Plain Node.js HTTP router mounted at /analytics/cohorts. Six routes (see below).
backend/analytics/__tests__/cohortAnalyticsRouter.test.ts	17 unit tests covering all routes, validation, and edge cases.

Modified:

• backend/server.ts — import + route block for /analytics/cohorts, placed before the 404 fallback.

Endpoints

Method	Path	Description
GET	/analytics/cohorts?merchantId=…&granularity=month|week	Cohort table grouped by subscriber signup period
GET	/analytics/cohorts/retention?merchantId=…	Day 1/7/30/60/90 retention curve
GET	/analytics/cohorts/churn?merchantId=…&from=<ms>&to=<ms>	Revenue churn vs logo churn breakdown
GET	/analytics/cohorts/export?merchantId=…&granularity=…&format=csv|pdf	Downloadable cohort report
GET	/analytics/cohorts/mrr?merchantId=…	MRR / ARR / ARPU / LTV summary
POST	/analytics/cohorts/subscribers	Upsert a subscriber record (seed the in-memory store)

Design decisions

• No new business logic. CohortService, AnalyticsDashboardApi, RetentionCalculator, and cohortReportExport already existed with full cohort-by-signup, retention curve, churn breakdown, and export support. This PR is purely a wiring layer.
• Export formats. CSV and PDF are both supported via the existing cohortTableToCsv / cohortTableToPdf helpers. The ?format= query param selects between them; CSV is the default.
• Granularity. month (default) and week are both supported; weekly cohorts use ISO-8601 week keys.
• Subscriber data seeding. POST /analytics/cohorts/subscribers lets callers upsert records into the in-memory SubscriberRecordRepository, making the endpoint self-contained until a database-backed repository is wired in.

Acceptance criteria (from issue)

• Cohorts by signup — cohort table grouped by month or week, keyed by signup period
• Exportable — CSV and PDF download via /export?format=csv|pdf
• Tested — 17 unit tests, all passing

Testing

npx jest --config jest.backend.config.js \
  --testMatch '**/backend/analytics/__tests__/cohortAnalyticsRouter.test.ts'

Test Suites: 1 passed
Tests:       17 passed, 0 failed

Covers: non-matching path fallthrough, missing merchantId → 400, wrong method → 405, cohort table (monthly + weekly), retention curve fields, churn breakdown, MRR/ARR/ARPU/LTV, CSV/PDF export headers and body, subscriber upsert and invalid JSON.

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