Phase 2: retention for per-user exports (occ backup:retention --user-last)

PruneService::userExportRetention()/setUserExportRetention(): a single, app-wide "keep the
newest N" setting (default 3, same default as the snapshot policy's "last"), deliberately simpler
than the full daily/weekly/monthly RetentionPolicy -- per-user exports are on-demand only (occ
backup:backup:user, see the fold-out task about whether they ever join the automatic schedule),
so there is no regular cadence yet to bucket by day/week/month.

PruneService::applyUserExports(): groups a location's user exports by uid (read from each
manifest's own 'time' field, the same field snapshot retention already reads), keeps the newest N
per user, forgets the rest. No trash, unlike a snapshot: these are not yet the primary safety net
the deletion delay protects. Runs under the same exclusive lock as snapshot retention.

occ backup:retention gained --user-last=N; occ backup:prune now also applies it for every target
it processes, reporting counts the same way it already does for snapshots and blob packs.

Verified live against the dev rig (four real exports across four users, confirmed exactly the
expected ones were kept/forgotten at two different --user-last values) and added coverage to
tests/integration/user-roundtrip.sh (an account's manual export plus the automatic safety-export
a replace-mode restore makes of it, pruned down to --user-last=2).

tests/integration/user-roundtrip.sh

@@ -54,6 +54,18 @@ SUM_AFTER=$($X sh -c "cd $DATA/$U/files && find . -type f -print0 | sort -z | xa
 [ "$SUM_BEFORE" = "$SUM_AFTER" ] || fail "files differ after a replace restore"
 ok "replace restores the original content under the same uid, byte-identical"
 
+# Retention: $U now has the manual export above plus the automatic safety-export the replace
+# restore made of it just before deleting it (4 total after these two more), then --user-last=2
+# must forget exactly the 2 oldest.
+sleep 1.1 # exportId is time-based to the second; force each one to be distinct
+occ backup:backup:user ci "$U" >/dev/null || fail "third backup:backup:user"
+sleep 1.1
+occ backup:backup:user ci "$U" >/dev/null || fail "fourth backup:backup:user"
+occ backup:retention --user-last=2 >/dev/null || fail "retention --user-last"
+PRUNE_OUT=$(occ backup:prune ci)
+echo "$PRUNE_OUT" | grep -q "forgot 2 user export(s) for 1 user(s)" || fail "prune did not forget the expected two user exports"
+ok "user-export retention forgets exactly the exports over the keep count"
+
 occ user:delete "$U" >/dev/null
 $X sh -c "rm -rf /tmp/ngb-urt-target /tmp/ngb-urt-pass"
 echo "USER ROUNDTRIP PASS"