feat(notifications): time-based notification jobs

[raymondjacobson]

Summary

Adds the three timer-driven notification creators the legacy Python discovery-provider celery beat produced, as scheduled parity jobs in the core-indexer. This closes the remaining functional gap between the vendored Go ETL indexer and the Python indexer for notifications: event-driven notifications are already handled by DB triggers (#851/#870), and these cover the ones that fire on elapsed time rather than an indexed entity.

• EngagementNotificationsJob (claimable_reward): promotes 7-day-cooldown challenges to claimable_reward once their cooldown elapses and they're still undisbursed. Complements the handle_on_user_challenge trigger, which only emits an immediate claimable_reward for cooldown_days = 0 and reward_in_cooldown for cooldown_days > 0. Hence the cooldown_days = 7 filter, matching Python's hardcoded check. Scheduled every 10m.
• ListenStreakReminderJob (listen_streak_reminder): reminds users in the 42-43h window after their last listen (6h of slack before the 48h streak breaks). Tight 1-2m window under env=stage for end-to-end testing, matching Python. Scheduled every 10s.
• RemixContestNotificationsJob: the four fan/artist remix_contest ended / ending_soon notifications, with the same audience rules (remixers, host followers, parent-track favoriters, event subscribers; host excluded from fan types). Scheduled every 30s.

Each job is a set-based INSERT ... SELECT ... ON CONFLICT (group_id, specifier) DO NOTHING for idempotency via uq_notification, and is wired into startParityJobs. Mirrors the corresponding apps/ Python tasks.

Test plan

• go test ./jobs/... — new tests cover the engagement full pipeline + exclusions (cooldown/disbursement/not-elapsed/incomplete), listen-streak window boundaries + idempotency, and remix-contest ended/ending-soon audiences with host exclusion.
• Tests account for the handle_on_user_challenge and handle_event triggers that also fire on seed (verified no overlap with the asserted notification types).
• go vet ./jobs/... ./indexer/... clean; full jobs package passes.
• Observe in stage that the jobs emit the expected notifications on the real schedule.

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