Tasks 12+13 of Phase 2 — first new infra after rename.
Alembic 008 creates scout_triage_queue with unique constraint on
(scout_id, source_msg_ref) and partial index on expires_at for active
rows. Adds four columns to cloud_scout_configs: auto_trash_spam,
gmail_history_id, gmail_watch_expires_at, device_inactivity_pause_days.
SQLAlchemy model ScoutTriageQueue added; CloudScoutConfig updated to
match. Imports extended with UniqueConstraint and text.
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