A former US Postal Service employee pleaded guilty in federal court Thursday to drug distribution charges, the US attorney’s office said. Sean Williamson, 42, who worked out of the Somerville Post Office, teamed with at least one other employee to distribute cocaine, US Attorney Carmen M. Ortiz’s office said. The pair were caught on video and audio recordings conducting drug transactions on several occasions in 2011, Ortiz’s office said. Williamson faces up to 20 years in prison. He is to be sentenced July 17.