Daily audio cron: extract decide_generation — table as code

[mircealungu]

Summary

Lifts the per-user generation decision (the table I keep drawing in chat
diagrams) into a single read-only function — one return per row.

def decide_generation(user, language, timezone_offset, today_local):
    sub = DailyAudioSubscription.find(user, language)
    if sub is None or not sub.enabled:                          return ("not-subscribed", None)
    if sub.lesson_type not in VALID_LESSON_TYPES:               return ("invalid-type",  sub)
    if not sub.scheduled_on(today_local):                       return ("not-due",       sub)
    if generator.today_lesson_exists(user, timezone_offset):    return ("exists",        sub)
    if DailyAudioLesson.waiting_paused_for(user, language.id):  return ("paused",        sub)
    return ("ready", sub)

The cron's main loop becomes a dispatch on kind, and generate_for_user does
only the heavy work (it no longer re-checks the gates — prepare_lesson_generation
still has its own existing-lesson check as belt-and-suspenders). Both dry-run
and real-run share the same decision, so the buckets can't drift between them.

Test plan

• python -m tools.generate_daily_audio_lessons --dry-run --days 30 — same buckets as before.
• --user-id <yours> real run still generates / skips as before.

Pairs with web#TBD (Today view's selectTodayView projection — same pattern on the client).

🤖 Generated with Claude Code

[github-actions]

ArchLens - No architecturally relevant changes to the existing views