Accuracy fixes for QoE, watch time, and engagement scores

We have improved accuracy across several QoE and engagement metrics in the processing pipeline. Customers may see small shifts in these metrics going forward as the calculations now better reflect real viewer behavior. Specific improvements:

1. Startup score — now correctly reflects instant-start sessions where videoStartupTime is 0.
2. Watch time — capped against the underlying video duration so reported values stay within realistic bounds.
3. Viewer time on dropped views — viewer time (vt) is now accurately captured for sessions that ended via a drop event.
4. Upscaling / downscaling percentages — both are now computed against the correct denominator, producing accurate values across all viewer-count ranges.
5. Overall experience score — now bounded within the expected range across all breakdown views.
6. Playback success score — improved propagation across downstream APIs for consistent reporting.
7. Browser name detection — improved User-Agent parsing so browser names are correctly identified.