Get sequence frame rates (fps)

Returns a map of sequence filename => fps for every .fseq file in the sequence directory. This is intentionally split out from the main file listing so the file manager can render the sequence list immediately and lazily fill in the FPS column via this endpoint. The fps is derived from the fseq header StepTime (fps = round(1000 / StepTime)) and cached per file (keyed on name + size) so fsequtils is only run on cache misses.