This small port town offers a textbook case of how Russian President Vladimir Putin has thwarted U.S. and European efforts to rein in Moscow’s most powerful source of leverage and cash: energy. Karlshamn’s local leaders in January opened its port to Russia’s state-owned energy company, PAO Gazprom, in defiance of Swedish national authorities alarmed by a growing Russian military presence in the Baltic Sea. A subcontractor for a Gazprom subsidiary is now allowed to store pipes here for an $11 billion…