docs: Add Mixpanel analytics configuration and documentation

- Add MIXPANEL_TOKEN, MIXPANEL_API_SECRET, ANALYTICS_ENV to .env.example
- Add packages/web/.env.example with VITE_MIXPANEL_TOKEN and other analytics vars
- Update docs/MIXPANEL_ANALYTICS.md with complete setup instructions
- Document event taxonomy (30+ events across User, Subscription, DarkWatch, VoicePrint, SpamShield)
- Add KPI definitions (MAU, MRR, conversion, churn, CAC, LTV, NPS, viral coefficient)
- Include integration examples for backend and frontend usage
- Document alert thresholds for monitoring

Implementation was already complete in packages/shared-analytics and packages/web.
This completes the configuration and documentation for Mixpanel setup.

FRE-5281

.env.example

@@ -27,3 +27,41 @@ SENTRY_TRACES_SAMPLE_RATE="0.1"
 # Google Analytics 4
 GA4_MEASUREMENT_ID=""
 GA4_API_SECRET=""
+
+# Mixpanel Product Analytics
+MIXPANEL_TOKEN=""
+MIXPANEL_API_SECRET=""
+ANALYTICS_ENV="development"
+
+# ============================================
+# Push Notifications Configuration
+# ============================================
+
+# Firebase Cloud Messaging (FCM) - Android
+FCM_PROJECT_ID=""
+FCM_CLIENT_EMAIL=""
+FCM_PRIVATE_KEY=""
+
+# Apple Push Notification Service (APNs) - iOS
+APNS_KEY_ID=""
+APNS_TEAM_ID=""
+APNS_BUNDLE_ID=""
+APNS_KEY=""
+
+# Twilio - SMS (optional)
+TWILIO_ACCOUNT_SID=""
+TWILIO_AUTH_TOKEN=""
+TWILIO_MESSAGING_SERVICE_SID=""
+
+# Notification Rate Limits
+PUSH_RATE_LIMIT=100
+EMAIL_RATE_LIMIT=60
+SMS_RATE_LIMIT=30
+RATE_LIMIT_WINDOW_SECONDS=60
+
+# Frontend Environment Variables (Vite)
+# Add these to packages/web/.env or your frontend .env files:
+# VITE_MIXPANEL_TOKEN=<same-as-backend-token>
+# VITE_GA_MEASUREMENT_ID=<same-as-backend-id>
+# VITE_META_PIXEL_ID=""
+# VITE_LINKEDIN_PARTNER_ID=""