Chore: Drop ImageMagik as a dependency (#14440)

* Mark the thumbnail constants Final

* Comment each pdftoppm option

* Feature/Chore: Replaces all usages of ImageMagik's convert binary with a simple Pillow based helper

This removes ImageMagik as a dependency entirly and all documentation is cleaned up from it.
We no longer need to install it, have a policy for it to allow PDFs and no longer shell
out to it either.

* Fix: Save WebP images losslessly when removing the alpha channel

Pillow's default WebP save is lossy, so a WebP with an alpha channel came
back from remove_alpha_channel with slightly shifted pixel values before
it reached OCR. Save WebP lossless; other formats are unchanged.

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

---------

Co-authored-by: Claude Sonnet 5.5 <noreply@anthropic.com>

src/documents/utils.py

@@ -9,11 +9,13 @@ from pathlib import Path
 from subprocess import CompletedProcess
 from subprocess import run
 from typing import TYPE_CHECKING
+from typing import Any
 from typing import Generic
 from typing import TypeVar
 
 from django.conf import settings
 from PIL import Image
+from PIL import ImageSequence
 
 if TYPE_CHECKING:
     from django.db.models import Model
@@ -128,6 +130,32 @@ def maybe_override_pixel_limit() -> None:
         Image.MAX_IMAGE_PIXELS = pixel_count
 
 
+def remove_alpha_channel(source: Path, dest: Path) -> Path:
+    """
+    Writes a copy of the image with its alpha channel dropped, keeping the
+    format, every frame, the DPI and any ICC profile.
+
+    The channel is dropped, not composited, so transparent pixels keep their
+    underlying color. Returns dest.
+    """
+    with Image.open(source) as im:
+        image_format = im.format
+        opaque_mode = "L" if im.mode == "LA" else "RGB"
+        frames = [frame.convert(opaque_mode) for frame in ImageSequence.Iterator(im)]
+        save_kwargs: dict[str, Any] = {}
+        if "dpi" in im.info:
+            save_kwargs["dpi"] = im.info["dpi"]
+        if "icc_profile" in im.info:
+            save_kwargs["icc_profile"] = im.info["icc_profile"]
+        if image_format == "WEBP":
+            save_kwargs["lossless"] = True
+        if len(frames) > 1:
+            save_kwargs["save_all"] = True
+            save_kwargs["append_images"] = frames[1:]
+    frames[0].save(dest, format=image_format, **save_kwargs)
+    return dest
+
+
 def run_subprocess(
     arguments: list[str],
     env: dict[str, str] | None = None,