v0.237.0: the Update button takes a backup first, and tells the truth (update arc slice 4 — R-448, R-443, R-439)

POST /api/stacks/{name}/update is now a guarded job answering 202:
cheap refusals (hold — R-439, busy, migration, deploying, memory via the
deploy's own memoryVerdict, a fixed 2 GB disk floor, and no restorable
Tier-2 copy) → backup-first when the proven copy is older than
update.backup_max_age (24h) → safety dump BEFORE the pin moves → pin →
pull (failure puts the pin back) → up → health (.felhom.yml check or 60 s
settle, update.health_timeout 5m). Not healthy → the app is stopped and
HELD (RestoreHold reason update_failed, same store and gate as R-379) and
the page names the backup to restore from; the pin stays. Success is only
ever update_phase=done after health (R-443). UpdateStack is deleted.

The restorable-unit predicate is EXTRACTED to backup.Tier2UnitRestorePoint
and shared with the backups page (row pinned unchanged). The copy is aged
by the last successful Tier-2 copy, not the manifest created_at — measured
on demo-hp that created_at moves only on definition changes.

Crash safety: update-journal.json before each phase; RecoverUpdates before
the boot sweep, ResumeInterruptedUpdates after the guards are wired.

Three unattended start paths ignored a hold and now honour it: the
drive-return gate (restart + boot recreate) and the nightly volume dump.
The nightly capture and Tier-2 run skip held apps so the restore point
survives. No automatic rollback — measured per-app; route back = restore.

Tests A–H across stacks/backup/api/web/cmd; six red-proofs seen to fail.

Claude-Session: https://claude.ai/code/session_0159rPz1ZhFKsS53msqPYxtS

controller/internal/backup/recovery_unit.go

@@ -385,6 +385,14 @@ func (m *Manager) captureAllRecoveryUnits() {
 		if m.settings != nil && (m.settings.IsDisconnected(drivePath) || m.settings.IsDecommissioned(drivePath)) {
 			continue // drive not writable — skip, the existing unit stays as-is
 		}
+		// Slice 4: a HELD app's unit is its RESTORE POINT, and the hold text names that copy's date.
+		// Re-capturing it would write the definition the app is held ON (after a failed update: the new
+		// version that would not start) into the unit, and the next Tier-2 run would mirror it over the
+		// copy the customer was told to restore from. The unit stays exactly as it was until the hold is
+		// lifted.
+		if m.isHeld(stack.Name) {
+			continue
+		}
 		m.noteAttempted(stack.Name)
 		// The reserve, checked BEFORE anything is written. Per app, and the loop continues.
 		if !m.admitApp(stack.Name) {