plakar-incus-backup: self-heal a silently expelled incus plugin

plakar's pkg backend deletes an installed ptar when its cache
re-extraction fails (observed 2026-08-24: orphaned .incus-* temps,
final ptar unlinked, 'unsupported importer protocol'). Reinstall from
the kept ptar before backing up instead of failing the night.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

plakar-incus-backup.sh

@@ -52,6 +52,18 @@ export PLAKAR_PASSPHRASE
 
 rc=0
 
+# The plakar pkg backend has been observed to silently expel an
+# installed plugin (its reload path deletes the ptar when the cache
+# re-extraction fails — seen 2026-08-24). Self-heal from the kept
+# ptar instead of failing the nightly run with
+# "unsupported importer protocol".
+if ! plakar pkg list 2>/dev/null | grep -q "^incus@"; then
+    PTAR=$(ls -t /root/incus-plugin/incus_v*.ptar 2>/dev/null | head -1)
+    echo "incus plugin missing — reinstalling ${PTAR:-<no ptar found>}" >&2
+    [ -n "$PTAR" ] && plakar pkg add "$PTAR" \
+        || { echo "incus plugin reinstall failed" >&2; exit 1; }
+fi
+
 log "backup @$SOURCE -> $KLOSET"
 plakar -quiet at "$KLOSET" backup -tag "$SOURCE" "@$SOURCE" \
     || { echo "backup @$SOURCE failed" >&2; rc=1; }