SEATTLE — As dreaded as “Thursday Night Football” games are for quick turnarounds, the 49ers used their turn to showcase why they own the NFL’s No. 1 defense — and the NFC West title. They also showed why rookie quarterback Brock Purdy is savvy enough to direct their offense into the playoffs, with the help of George Kittle (two touchdown catches), Christian McCaffrey (one touchdown, 138 total…