feat(media): sweep removes the folders a completed run emptied

An apply leaves every release folder it emptied standing where it was: the
live library run left 1675 husks at the root. sweep takes the same journal
and deletes only folders it can prove this run emptied - a journalled move
out of the folder or something under it, nothing but empty directories left,
no journalled destination beneath it, not a folder the run created. A folder
nobody moved cannot be selected at all, so a hand-made placeholder survives,
and a symlink anywhere inside disqualifies the subtree. Every rmdir is
journalled, so rollback recreates what a sweep removed.

tests/media_relayout.bats

@@ -331,3 +331,105 @@ PY
     # the refusal is all-or-nothing: another film did not move back
     [ -e "$LIB/Marnie (1964)/Marnie (1964).mkv" ]
 }
+
+# ---------------------------------------------------------------- sweep
+#
+# A relayout leaves the release folders it emptied standing at the library root:
+# 1675 of them after the live run, each holding nothing but empty subfolders. The
+# sweep pass removes only folders THIS RUN emptied — proven from the journal,
+# never from "it looks empty" — so a folder someone made by hand is untouchable,
+# and every removal rides the same rollback as the moves.
+
+@test "sweep removes only the folders this run emptied" {
+    plan
+    apply_plan
+    mkdir -p "$LIB/A Empty Placeholder"      # someone's folder, never a move source
+    run python3 "$APPLY" sweep --root "$LIB" --journal "$JOURNAL" --apply
+    [ "$status" -eq 0 ]
+    [[ "$output" == *"swept"* ]]
+    [ ! -e "$LIB/AMERICAN POP" ]
+    [ ! -e "$LIB/VHS 2 (2013)" ]
+    [ ! -e "$LIB/Alien (1979)/Featurettes" ]
+    [ ! -e "$LIB/Alien (1979)/subs" ]
+    [ ! -e "$LIB/Marnie (1964)/Exrtas" ]
+    # a folder the run never moved a file out of survives, empty or not
+    [ -d "$LIB/A Empty Placeholder" ]
+}
+
+@test "sweep without --apply reports the husks and changes nothing" {
+    plan
+    apply_plan
+    local before jlines
+    before=$(find "$LIB" -type d | wc -l)
+    jlines=$(wc -l < "$JOURNAL")
+    run python3 "$APPLY" sweep --root "$LIB" --journal "$JOURNAL"
+    [ "$status" -eq 0 ]
+    [[ "$output" == *"--apply"* ]]
+    [ "$(find "$LIB" -type d | wc -l)" -eq "$before" ]
+    [ "$(wc -l < "$JOURNAL")" -eq "$jlines" ]
+    [ -e "$LIB/AMERICAN POP" ]
+}
+
+@test "a folder that still holds content is never swept" {
+    plan
+    apply_plan
+    run python3 "$APPLY" sweep --root "$LIB" --journal "$JOURNAL" --apply
+    [ "$status" -eq 0 ]
+    # the film folder, the folder the run created, and the piles all stay
+    [ -d "$LIB/Marnie (1964)" ]
+    [ -e "$LIB/Marnie (1964)/Marnie (1964).mkv" ]
+    [ -d "$LIB/V-H-S-2 (2013)" ]
+    [ -d "$LIB/_unsorted bonus/extras" ]
+}
+
+@test "rollback after a sweep brings the folders and the files back" {
+    plan
+    apply_plan
+    run python3 "$APPLY" sweep --root "$LIB" --journal "$JOURNAL" --apply
+    [ "$status" -eq 0 ]
+    [ ! -e "$LIB/AMERICAN POP" ]
+    run python3 "$APPLY" rollback --root "$LIB" --journal "$JOURNAL" --apply
+    [ "$status" -eq 0 ]
+    [ -e "$LIB/AMERICAN POP/AMERICAN.POP.2023.1080p.WEB-DL.mkv" ]
+    [ -e "$LIB/VHS 2 (2013)/VHS.2.2013.1080p.BluRay.x265.mkv" ]
+    [ -e "$LIB/Alien (1979)/Featurettes/A Bug And A Bite - Making Alien.mkv" ]
+    [ -e "$LIB/Alien (1979)/subs/3_Eng.srt" ]
+}
+
+@test "a second sweep has nothing left to do and says so" {
+    plan
+    apply_plan
+    run python3 "$APPLY" sweep --root "$LIB" --journal "$JOURNAL" --apply
+    [ "$status" -eq 0 ]
+    local before
+    before=$(find "$LIB" -type d | wc -l)
+    run python3 "$APPLY" sweep --root "$LIB" --journal "$JOURNAL" --apply
+    [ "$status" -eq 0 ]
+    [[ "$output" == *"swept: 0"* ]]
+    [ "$(find "$LIB" -type d | wc -l)" -eq "$before" ]
+}
+
+@test "sweep never follows a symlink and never removes the folder it guards" {
+    plan
+    apply_plan
+    mkdir -p "$LIB/Elsewhere"
+    at "$LIB/Elsewhere/keep.txt" 10
+    ln -s "$LIB/Elsewhere" "$LIB/AMERICAN POP/pointer"
+    run python3 "$APPLY" sweep --root "$LIB" --journal "$JOURNAL" --apply
+    [ "$status" -eq 0 ]
+    # a link is content, not nothing: the husk it sits in survives untouched
+    [ -d "$LIB/AMERICAN POP" ]
+    [ -L "$LIB/AMERICAN POP/pointer" ]
+    [ -e "$LIB/Elsewhere/keep.txt" ]
+}
+
+@test "sweep refuses a journal that belongs to another library" {
+    plan
+    apply_plan
+    mkdir -p "$BATS_TEST_TMPDIR/Other"
+    run python3 "$APPLY" sweep --root "$BATS_TEST_TMPDIR/Other" \
+        --journal "$JOURNAL" --apply
+    [ "$status" -ne 0 ]
+    [[ "$output" == *"outside"* ]]
+    [ -d "$LIB/AMERICAN POP" ]
+}