Function run_once 

pub async fn run_once(pool: &PgPool, period_hours: u64) -> AppResult<u64>

Queues the digests that are due (every period_hours; 0 = never). Returns how many were queued.

Invariants

• Permission: none; a background task, not an actor. Only active editors and administrators with e-mail on, the kind not muted, who are members of an active group with new requests.
• Locks: the digest row of the user and group (FOR UPDATE), so an overlapping run (or another server) finds the period covered.
• Data: the digest lists only requests the user may read; the covered period moves on even when no digest text results.
• Audit: none.
• Notifications: one request_digest notification per due user and group, e-mail queued; webhooks: none.
• Atomic: one transaction per user and group (tests/request_digests.rs).