Harden follow-up delivery and mailroom persistence

[scgopi]

Summary

• retain failed follow-up deliveries for retry, with memory recovery and an error announcement
• recover deferred and in-flight follow-ups when a delivery lease expires without allowing the old drain to reinsert stale state
• update the mailroom digest only after graph and sidecar persistence succeeds

Validation

• swift build passes
• git diff --check passes
• focused Xcode tests were attempted but the generated project could not resolve IdentifiedCollections; tests did not execute

Fixes the three Beta6 release blockers: failed follow-ups, lease recovery ordering, and sidecar-write data loss.