Add timestamps to livestream URLs

We have one livestream URL per room and day, and post the same URL on every session notification. If the streams are started at a defined time (e.g. 8:30 AM, or 10 minutes before the first scheduled session), we could add timestamps to these session streaming URLs so users can just jump to the session.

No files or tests are named. Start by locating the code that builds session notifications and livestream URLs, then trace how each room/day URL is assigned to sessions. Done means session links include timestamps based on the defined stream start time, including the stated 8:30 AM or pre-session-start rule.

Written by the indexing model from the issue text.