Fix: keep counter and extension when shortening long public filenames (#14472)

get_public_filename relied on sanitize_filename to truncate, which cuts
from the end and so dropped the counter and extension of long titles.
Multi-byte titles could also exceed the 255 byte filesystem limit.

Shorten the title by UTF-8 bytes before appending the counter and
extension. The exporter reserves room for its longer derived thumbnail
and archive suffixes.

src/documents/tests/test_management_exporter.py

@@ -1537,3 +1537,70 @@ class TestExportDerivedFileNameCollisions:
             assert (
                 target / record[EXPORTER_ARCHIVE_NAME]
             ).read_text() == f"archive {pk}"
+
+    def test_long_titles_export_within_file_name_limit(
+        self,
+        paperless_dirs: PaperlessDirs,
+        tmp_path: Path,
+    ) -> None:
+        """
+        GIVEN:
+            - Two documents with the same created date and the same title
+            - The title alone is longer than 255 bytes and contains umlauts
+        WHEN:
+            - The exporter runs
+        THEN:
+            - The export finishes with distinct original, thumbnail and archive names
+            - Every exported file name fits in the byte limit
+            - Each exported file holds its own document's content
+        """
+        for pk in (1, 2):
+            (paperless_dirs.originals_dir / f"{pk:07}.pdf").write_bytes(
+                f"original {pk}".encode(),
+            )
+            (paperless_dirs.archive_dir / f"{pk:07}.pdf").write_bytes(
+                f"archive {pk}".encode(),
+            )
+            (paperless_dirs.thumbnail_dir / f"{pk:07}.webp").write_bytes(
+                f"thumbnail {pk}".encode(),
+            )
+            DocumentFactory(
+                pk=pk,
+                title="Bestellbestätigung " * 15,
+                created=date(2026, 10, 15),
+                filename=f"{pk:07}.pdf",
+                archive_filename=f"{pk:07}.pdf",
+                mime_type="application/pdf",
+            )
+
+        target = tmp_path / "export"
+        target.mkdir()
+        call_command(
+            "document_exporter",
+            str(target),
+            "--no-progress-bar",
+            skip_checks=True,
+        )
+
+        manifest = json.loads((target / "manifest.json").read_text())
+        records = {r["pk"]: r for r in manifest if r["model"] == "documents.document"}
+
+        for key in (
+            EXPORTER_FILE_NAME,
+            EXPORTER_THUMBNAIL_NAME,
+            EXPORTER_ARCHIVE_NAME,
+        ):
+            assert records[1][key] != records[2][key]
+
+        for pk in (1, 2):
+            record = records[pk]
+            for key, content in (
+                (EXPORTER_FILE_NAME, f"original {pk}"),
+                (EXPORTER_THUMBNAIL_NAME, f"thumbnail {pk}"),
+                (EXPORTER_ARCHIVE_NAME, f"archive {pk}"),
+            ):
+                assert (
+                    len(Path(record[key]).name.encode())
+                    <= Document.MAX_PUBLIC_FILENAME_BYTES
+                )
+                assert (target / record[key]).read_text() == content