The ornamental design for seat mechanism, as shown and described.
1. Seat mechanism
The model is a mechanism for a chair or an armchair; 1.1 is a bottom perspective view; 1.2 is a front perspective view; 1.3 is a top perspective view; 1.4 is a right side view; 1.5 is a left side view; 1.6 is a front view; 1.7 is a rear view; 1.8 is a bottom plan view.
In the drawings, the broken lines illustrate portions of the seat mechanism that form no part of the claimed design.