Sessionise plays with a 30-minute rule

Constraints

1. 1Sessions are per listener. Two listeners playing at the same moment are in different sessions.
2. 2A gap of more than 30 minutes starts a new session. Exactly 30 minutes does not.
3. 3A listener's first play always starts a session, so every listener has at least one.
4. 4plays_per_session is plays divided by sessions, rounded to 2 decimal places.
5. 5A calendar day is not a session — three listeners here have two sessions inside one day.
6. 6Order by user_id.

Worked example

u1 plays at 20:00, 20:04 and 20:08 on 5 January, then again at 21:10. The first three are four minutes apart, so they are one session; the 62-minute break opens a second. February and March add one session each, giving 7 plays across 4 sessions. Counting distinct days instead would report 3 — it merges those two separate evening sessions into one, and the plays-per-session figure inherits the error.

What this tests

The three-step sessionisation pattern — LAG for the gap, a 0/1 flag for a session start, then a running SUM of that flag to number the sessions. It is the standard answer to 'group consecutive rows into runs'.