Add user-scoped POST /api/sessions/:id/pause and /resume endpoints,
mirroring the stop handler's ownership/lookup and 401/404/409 guards.
Pause sets paused_at (status stays active); resume folds the open span
into paused_seconds and clears paused_at.
Change stop to fold any open pause span into paused_seconds, then set
duration_seconds = max(0, round((end-start)/1000) - paused_seconds) so
saved duration is worked time, and clear paused_at.
Add a "Paused Duration" column to /api/export (after "Total Duration")
using formatDuration(paused_seconds).
Products affected: SoleLog backend (apps/api)
