CVE-2026-114582: Server-side request forgery via `SpatialReference` string input

`SpatialReference` passed string input to GDAL,

which by default resolves URLs and reads files. This behavior was not
documented by Django. Other affected methods included
`GEOSGeometry.transform()`, `OGRGeometry.transform()`, `GDALRaster.transform()`,
the `srs` setter of `OGRGeometry`, and the `srs` and `srid` setters of `GDALRaster`.
A URL or file path reaching any of these would issue an outbound request or
read a file as the Django process user. The attack vectors could be combined to
reveal local file contents by observing outbound requests.

`SpatialReference` now disables network and file access when parsing string
input on GDAL 3.9 and later. As a consequence, file paths and URLs are no
longer accepted as `SpatialReference` inputs. Dictionary references using the
`DICT:` prefix, including those prefixed with `ESRI::`, are rejected on all
GDAL versions.

Full protection requires GDAL 3.9 or later. Integer SRIDs are looked up
directly in the EPSG database, so prefer them when the input is untrusted. Or,
if strings must be used on lower GDAL versions, compare to an allow list. The
new `gis.W001` system check warns when the installed version of GDAL is not
at least 3.9.

This issue has severity "low" according to the [Django security policy](https://docs.djangoproject.com/en/dev/internals/security/#security-issue-severity-levels).

Thanks adraca for the report.