Add NoOp email service, /docs shortcut and fix streak timezone bug

Replace nil email service pattern with NoOpEmailService (Null Object)
to eliminate nil pointer panics across all handlers.

Add /docs route as a shortcut to Swagger UI.

Fix streak calculation returning 0 when server timezone differs from
UTC — CreatedAt was not converted to UTC before date extraction.

cmd/api/main.go

@@ -95,7 +95,7 @@ func main() {
 	jwtService := auth.NewJWTService(cfg.JWTSecret, jwtExpiryHours)
 	passwordHasher := crypto.NewBcryptHasher()
 
-	var emailService services.EmailService
+	var emailService services.EmailService = &services.NoOpEmailService{}
 	if cfg.SMTPHost != "" {
 		smtpPort, err := strconv.Atoi(cfg.SMTPPort)
 		if err != nil {