Refs #748 - Serialize storage alias whenever possible

[magicalyak]

Summary

This PR fixes issue #748 where users experience storage errors with Django 4.2+ when using cloud storage backends (S3, Google Cloud Storage, etc.).

The Problem

Users reported errors like:

ValueError: Cannot determine path without bucket name.

When using Django's STORAGES configuration with OPTIONS (bucket names, credentials, etc.), sorl-thumbnail would fail because storage instances were created without these required OPTIONS.

Real-world impact: Projects like NetBox are blocked by this issue (netbox-community/netbox#20212), unable to use S3 storage with sorl-thumbnail until this fix is released.

Root Cause

While commit b348c63 added support for Django storage aliases in get_or_create_storage(), the serialization still used the backend class path instead of the alias. This meant:

1. ✅ Create thumbnail → Uses storages["default"] with OPTIONS
2. ❌ Serialize → Saves "storages.backends.s3.S3Storage" (class path)
3. ❌ Deserialize → Calls get_module_class()() without OPTIONS → FAILS

As @panizza noted in issue #748:

"When the serialize_image_file method is called you serialize the storage path, not the storage alias... the deserialize_image_file retrieves the saved storage path and create a new storage without options, leading to errors."

The Solution

This PR completes the fix by ensuring storage aliases are serialized whenever possible:

1. ✅ Create thumbnail → Uses storages["default"] with OPTIONS
2. ✅ Serialize → Saves "default" (alias)
3. ✅ Deserialize → Uses storages["default"] with OPTIONS → SUCCESS

Changes

• Modified serialize_storage() to return the storage alias (e.g., "default") instead of the backend class path when the alias exists in Django's STORAGES configuration
• Updated test expectations to match new serialization format
• Rebased PR Refs #748 - Serialize storage alias whenever possible #780 by @claudep onto latest master to fix CI issues
• Added imagemagick to GitHub workflow dependencies

Related Issues

• Fixes django 4.2 - storage options (problem with s3 and template tag) #748 - Django 4.2 storage options problem with S3 and template tag
• Fixes Thumbnails don't work with S3 storage netbox-community/netbox#20212 - NetBox thumbnails fail with S3 storage
• Alternative to PR Refs #748 - Serialize storage alias whenever possible #780 (rebased with latest master changes)

Test Plan

• All existing tests updated to reflect new serialization format
• test_storage_serialize passes - directly validates storage alias serialization
• Tests pass with Python 3.9 and 3.13
• Code follows project style (ruff linting passes)
• Coverage requirements met

Testing with Real S3 Configuration

This fix allows users to configure S3 storage like:

STORAGES = {
    "default": {
        "BACKEND": "storages.backends.s3.S3Storage",
        "OPTIONS": {
            "bucket_name": "my-bucket",
            "access_key": "...",
            "secret_key": "...",
            # ... other OPTIONS
        },
    },
}

And thumbnails will now correctly serialize/deserialize using the "default" alias, preserving all OPTIONS through the entire lifecycle.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 64.05%. Comparing base (0df1039) to head (b3c24c4).
⚠️ Report is 3 commits behind head on master.

❗ There is a different number of reports uploaded between BASE (0df1039) and HEAD (b3c24c4). Click for more details.

HEAD has 27 uploads less than BASE

Flag	BASE (0df1039)	HEAD (b3c24c4)
	30	3

Additional details and impacted files

@@             Coverage Diff             @@
##           master     #788       +/-   ##
===========================================
- Coverage   74.07%   64.05%   -10.02%     
===========================================
  Files          26       26               
  Lines        1678     1686        +8     
===========================================
- Hits         1243     1080      -163     
- Misses        435      606      +171     

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[claudep]

Now that my PR for #748 was merged, maybe you can rebase so that we can push your commit?

[magicalyak]

Now that my PR for #748 was merged, maybe you can rebase so that we can push your commit?

Looks like the only change this adds is for the CHANGES.rst so I'm thinking I can close this. Thanks so much for adding the changes, I really appreciate it! @claudep

[magicalyak]

@claudep not sure it's work a PR just for that but feel free to close if not (or let me know and I will).

[claudep]

Thanks! Documentation is precious, too!