Production hardening: security, resilience, observability, and compliance

Password complexity: custom validator requiring uppercase, lowercase, digit (min 8 chars)
Token expiry: 90-day token lifetime with refresh endpoint (60-90 day renewal window)
Health check: /api/health/ now pings Postgres + Redis, returns 503 on failure
Audit logging: async audit_log table for auth events (login, register, delete, etc.)
Circuit breaker: APNs/FCM push sends wrapped with 5-failure threshold, 30s recovery
FK indexes: 27 missing foreign key indexes across all tables (migration 017)
CSP header: default-src 'none'; frame-ancestors 'none'
Gzip compression: level 5 with media endpoint skipper
Prometheus metrics: /metrics endpoint using existing monitoring service
External timeouts: 15s push, 30s SMTP, context timeouts on all external calls

Migrations: 016 (token created_at), 017 (FK indexes), 018 (audit_log)
Tests: circuit breaker (15), audit service (8), token refresh (7), health (4),
       middleware expiry (5), validator (new)

internal/validator/validator.go

@@ -4,6 +4,7 @@ import (
 	"net/http"
 	"reflect"
 	"strings"
+	"unicode"
 
 	"github.com/go-playground/validator/v10"
 	"github.com/labstack/echo/v4"
@@ -27,9 +28,34 @@ func NewCustomValidator() *CustomValidator {
 		return name
 	})
 
+	// Register custom password complexity validator
+	v.RegisterValidation("password_complexity", validatePasswordComplexity)
+
 	return &CustomValidator{validator: v}
 }
 
+// validatePasswordComplexity checks that a password contains at least one
+// uppercase letter, one lowercase letter, and one digit.
+// Minimum length is enforced separately via the "min" tag.
+func validatePasswordComplexity(fl validator.FieldLevel) bool {
+	password := fl.Field().String()
+	var hasUpper, hasLower, hasDigit bool
+	for _, ch := range password {
+		switch {
+		case unicode.IsUpper(ch):
+			hasUpper = true
+		case unicode.IsLower(ch):
+			hasLower = true
+		case unicode.IsDigit(ch):
+			hasDigit = true
+		}
+		if hasUpper && hasLower && hasDigit {
+			return true
+		}
+	}
+	return hasUpper && hasLower && hasDigit
+}
+
 // Validate implements echo.Validator interface
 func (cv *CustomValidator) Validate(i interface{}) error {
 	if err := cv.validator.Struct(i); err != nil {
@@ -96,6 +122,8 @@ func formatMessage(fe validator.FieldError) string {
 		return "Must be a valid URL"
 	case "uuid":
 		return "Must be a valid UUID"
+	case "password_complexity":
+		return "Password must be at least 8 characters with at least one uppercase letter, one lowercase letter, and one digit"
 	default:
 		return "Invalid value"
 	}