Feature: Generate PDF thumbnails via poppler utils (#14420)

* Chore: Generate PDF thumbnails with pdftoppm and Pillow instead of ImageMagick

Replace the ImageMagick convert based PDF thumbnail path with pdftoppm and
Pillow. Keeps thumbnails at 500px wide for small and odd sized pages, covers
the qpdf repair and double failure paths, removes the unused run_convert
helper, hardens thumbnail encoding, deprecates PAPERLESS_CONVERT_MEMORY_LIMIT
and PAPERLESS_CONVERT_TMPDIR, and corrects the related docs and CI.

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>

* Chore: Supersample the PDF thumbnail render 2x

Render the PDF page at twice the target size and downsample, with edge case
handling for small and odd sized pages, simplified helpers and tests, and
trimmed comments.

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>

* Fix: use a fractional thumbnail DPI so oversized pages render a bounded raster

* Deprecate the unused PAPERLESS_GS_BINARY setting too

* Warn at startup when pdftoppm is missing

* Resample PDF thumbnails once instead of twice with LANCZOS, still never enlarging

* Read PDF page size from pdfinfo so it matches pdftoppm

* Mark the thumbnail constants Final

* Comment each pdftoppm option

src/paperless/tests/parsers/test_tesseract_parser.py

@@ -15,9 +15,10 @@ from typing import TYPE_CHECKING
 
 import pytest
 from ocrmypdf import SubprocessOutputError
+from PIL import Image
 
 from documents.parsers import ParseError
-from documents.parsers import run_convert
+from documents.parsers import rasterize_pdf_page_to_png
 from paperless.models import ModeChoices
 from paperless.parsers import ParserProtocol
 from paperless.parsers.tesseract import RasterisedDocumentParser
@@ -280,24 +281,71 @@ class TestGetThumbnail:
         )
         assert thumb.is_file()
 
-    def test_thumbnail_fallback_on_convert_error(
+    @pytest.mark.parametrize(
+        ("filename", "expected_height"),
+        [
+            pytest.param("simple-digital.pdf", 647, id="portrait-letter"),
+            pytest.param("rotated.pdf", 386, id="landscape"),
+        ],
+    )
+    def test_thumbnail_is_correct_format_and_size(
+        self,
+        tesseract_parser: RasterisedDocumentParser,
+        tesseract_samples_dir: Path,
+        filename: str,
+        expected_height: int,
+    ) -> None:
+        """
+        GIVEN:
+            - A portrait or landscape PDF
+        WHEN:
+            - A thumbnail is generated
+        THEN:
+            - A 500px wide WebP keeping the page's aspect ratio
+        """
+        thumb = tesseract_parser.get_thumbnail(
+            tesseract_samples_dir / filename,
+            "application/pdf",
+        )
+        with Image.open(thumb) as im:
+            assert im.format == "WEBP"
+            assert im.width == 500
+            assert im.height == pytest.approx(expected_height, abs=2)
+
+    def test_thumbnail_fallback_on_pdftoppm_error(
         self,
         mocker: MockerFixture,
         tesseract_parser: RasterisedDocumentParser,
         tesseract_samples_dir: Path,
     ) -> None:
-        def _raise_on_pdf(input_file, output_file, **kwargs) -> None:
-            if ".pdf" in str(input_file):
+        """
+        GIVEN:
+            - Rasterizing the original PDF fails
+        WHEN:
+            - A thumbnail is generated
+        THEN:
+            - The PDF is repaired with qpdf and a real thumbnail is rendered
+        """
+        original = tesseract_samples_dir / "simple-digital.pdf"
+
+        def _fail_on_original(in_path: Path, out_path: Path, **kwargs) -> None:
+            if in_path == original:
                 raise ParseError("Does not compute.")
-            run_convert(input_file=input_file, output_file=output_file, **kwargs)
+            rasterize_pdf_page_to_png(in_path, out_path, **kwargs)
 
-        mocker.patch("documents.parsers.run_convert", side_effect=_raise_on_pdf)
-
-        thumb = tesseract_parser.get_thumbnail(
-            tesseract_samples_dir / "simple-digital.pdf",
-            "application/pdf",
+        rasterize = mocker.patch(
+            "documents.parsers.rasterize_pdf_page_to_png",
+            side_effect=_fail_on_original,
         )
+
+        thumb = tesseract_parser.get_thumbnail(original, "application/pdf")
+
+        assert rasterize.call_count == 2
         assert thumb.is_file()
+        assert thumb.name == "convert_qpdf.webp"
+        with Image.open(thumb) as im:
+            assert im.format == "WEBP"
+            assert im.width == 500
 
     def test_thumbnail_encrypted_pdf(
         self,