Streak: count cumulative daily activity, not just single 2-min sessions

[mircealungu]

Problem

The streak qualifies a day as "practiced" only when a single activity session reaches MIN_STREAK_DURATION_MS = 120_000 (2 min). See zeeguu/api/endpoints/helpers/activity_sessions.py:23-31.

This means two 1-minute sessions on the same day do not qualify, even though the user practiced for 2 minutes total. Realistic patterns where this hurts:

• A short replay of a past audio lesson + a short article read.
• Several brief listens spread across the day (commute, break, evening).
• An exercise round + a short lesson.

Proposal

Qualify the streak based on cumulative time across all of today's activity sessions in the user's learned language, rather than per-session duration.

Rough sketch:

• On any session update, sum today's UserListeningSession + UserReadingSession + UserExerciseSession durations for the user (filtered to the relevant language via the _session_language rule already in activity_sessions.py).
• If the sum >= MIN_STREAK_DAILY_MS (still 120_000, or possibly raised), call update_streak_if_needed.

Scope notes

• The MIN_STREAK_DURATION_MS constant becomes per-day, not per-session.
• One query per session-update is fine (sessions are short and (user_id, start_time) is indexed) — should not be a measurable cost.
• No frontend change required; this is purely backend qualification logic.

Related

Surfaced while wiring useListeningSession into PastLessons.js (web) so history-replay listening time is recorded. With that in place, the data is now there — but a user who replays for ~90 seconds still won't get streak credit until cumulative-day qualification is implemented.