feat: add funnel conversion tracking, ML forecasting, A/B testing, customer health (#852 #853 #854 #855)

[SagaciousBoss]

Closes #852
Closes #853
Closes #854
Closes #855

Summary

This PR implements all four requested features:

#852 Build A/B testing framework

• Service \�ackend/src/services/ab-testing.ts\ (\ABTestingService) with deterministic MD5 bucketing, weighted variants, traffic allocation, lifecycle (draft/running/paused/completed/archived)
• Statistical significance: two-proportion z-test (pooled), Wilson score interval, Bayesian Beta-Binomial, sample size calculator
• Routes \�ackend/src/routes/ab-testing.ts\ at /api/v1/ab-tests\
• Tests: \�ackend/src/services/tests/ab-testing.test.ts\ + docs \�ackend/docs/AB_TESTING.md\

#853 Add funnel conversion tracking

• Service \�ackend/src/services/funnel-tracking.ts\ (\FunnelTrackingService) — custom funnel definitions, step ordering, conversion window, per-step stats (entered/conversionRate/stepConversionRate/dropOff/avg/median/p95 time to next), user journey, CSV export
• Routes \�ackend/src/routes/funnel-tracking.ts\ at /api/v1/funnels\
• Tests + docs

#854 Implement revenue forecasting with ML

• Service \�ackend/src/services/ml-forecast.ts\ (\MLForecastService) — ensemble (linear, polynomial degree 2, exponential smoothing, Holt-Winters seasonal, moving average) with cross-validated model selection (80/20 holdout, RMSE), confidence intervals, seasonality detection via autocorrelation
• Routes \�ackend/src/routes/ml-forecast.ts\ at /api/v1/forecast/ml\

#855 Build customer health score system

• Service \�ackend/src/services/customer-health.ts\ (\CustomerHealthService) — composite 0-100 (payment 30% + frequency 20% + recency 20% + engagement 15% + support 15%) with churn penalties, levels champion/healthy/at_risk/critical, trends, at-risk listing, distribution
• Routes \�ackend/src/routes/customer-health.ts\ at /api/v1/customer-health\

All routers registered in \�ackend/src/index.ts.

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