feat(config): add feature flags for optional security features

- Add enablePasswordReset, enableRefreshTokenRotation, enableAuditLogging flags
- All flags default to false for backward compatibility
- Add getOptionalBoolean helper for parsing boolean env vars
- Update .env.example with feature flag documentation

backend/.env.example

@@ -2,4 +2,10 @@
 PORT=8000
 NODE_ENV=production
 DATABASE_URL=file:/app/prisma/dev.db
-FRONTEND_URL=http://localhost:6767
+FRONTEND_URL=http://localhost:6767
+
+# Optional Feature Flags (all default to false for backward compatibility)
+# Set to "true" or "1" to enable:
+# ENABLE_PASSWORD_RESET=false
+# ENABLE_REFRESH_TOKEN_ROTATION=false
+# ENABLE_AUDIT_LOGGING=false