fix(media): the census read Plex the same wrong way

The two Plex arms of media-census.py — `plex` (the movie planner's title
authority) and `audiobooks` (the chapter rows the audiobook plan files by) —
opened the library `mode=ro&immutable=1` while the Jellyfin arms already used
plain `mode=ro`. Same defect as media-verify.py: an uncheckpointed row of a
running scan is invisible, so a just-scanned item is simply absent from the
authority file a plan is built from, and the pass cannot say why.

Both now go through open_plex_ro(), which tries plain ro first and keeps
immutable only as the fallback for a database whose -shm cannot be read. The
new audiobooks test leaves a chapter row committed-but-uncheckpointed and
asserts the export still carries it; red before the change, green after.

tests/media_audiobooks.bats

@@ -228,6 +228,51 @@ PY
     [ "$(wc -l < "$BATS_TEST_TMPDIR/exp.tsv")" -eq 1 ]   # header only
 }
 
+# wal_fresh DB ROOT OUT — a row Plex matched during a scan and committed, but
+# has not checkpointed: the normal state of a library being walked. The holder
+# connection stops the WAL folding into the main file when the writer closes,
+# and the census runs as a real subprocess against those same files.
+wal_fresh() {
+    local db="$1" root="$2" out="$3"
+    python3 - "$db" "$root" "$out" "$CENSUS" <<'PY'
+import sqlite3, subprocess, sys
+db, root, out, census = sys.argv[1:5]
+w = sqlite3.connect(db)
+w.execute("pragma journal_mode=wal")
+w.execute("pragma wal_autocheckpoint=0")
+w.commit()
+w.execute("pragma wal_checkpoint(truncate)")     # the rows so far are on disk
+holder = sqlite3.connect(db)                     # last close would checkpoint
+ar = w.execute('insert into metadata_items (parent_id, metadata_type, title,'
+               ' year, "index", deleted_at) VALUES (NULL, 8, ?, NULL, NULL, NULL)',
+               ("Fresh Narrator",)).lastrowid
+al = w.execute('insert into metadata_items (parent_id, metadata_type, title,'
+               ' year, "index", deleted_at) VALUES (?, 9, ?, ?, NULL, NULL)',
+               (ar, "Fresh Book", 2026)).lastrowid
+mi = w.execute('insert into metadata_items (parent_id, metadata_type, title,'
+               ' year, "index", deleted_at) VALUES (?, 10, ?, NULL, ?, NULL)',
+               (al, "Fresh Chapter", 1)).lastrowid
+it = w.execute("insert into media_items (metadata_item_id, deleted_at)"
+               " VALUES (?, NULL)", (mi,)).lastrowid
+w.execute("insert into media_parts (media_item_id, file, deleted_at)"
+          " VALUES (?, ?, NULL)", (it, root + "/Fresh/01.mp3"))
+w.commit()
+sys.exit(subprocess.run([sys.executable, census, "audiobooks", "--root", root,
+                         "--db", db, "--out", out]).returncode)
+PY
+}
+
+@test "the census exports a row the scanner committed but has not checkpointed" {
+    DB="$BATS_TEST_TMPDIR/plex.db"
+    db_with_part "$DB" "$BATS_TEST_TMPDIR/Audiobooks/Old/01.mp3" \
+        "Old Narrator" "Old Book" 1990 1 "Old Chapter"
+    run wal_fresh "$DB" "$BATS_TEST_TMPDIR/Audiobooks" "$BATS_TEST_TMPDIR/exp.tsv"
+    [ "$status" -eq 0 ]
+    [ "$(wc -l < "$BATS_TEST_TMPDIR/exp.tsv")" -eq 3 ]   # header + both rows
+    tail -1 "$BATS_TEST_TMPDIR/exp.tsv" \
+        | grep -q "^$BATS_TEST_TMPDIR/Audiobooks/Fresh/01\.mp3	Fresh Narrator	Fresh Book	2026	1	Fresh Chapter$"
+}
+
 # -------------------------------------------------------------------- the plan
 
 @test "a matched chapter files under its narrator and its book, name untouched" {