SERIES FINALE: At 8 p.m. ET, the CW provides a retrospective special, trying to encapsulate and summarize, for long-standing fans and curious last-second viewers, exactly what the hell this vampire romance drama was all about. Basically, it was about two vampire brothers, played by Paul Wesley and Ian Somerhalder – but truly, it was about Nina Dobrev, who in her years with the show played no fewer than four roles: not only doppelganger rivals Katherine and Elena, but also lesser figures Tatia and Amara. Dobrev left the show in 2015 – but returns tonight for the finale. In how many roles, though, I have no idea…