Add Vercel Blob storage backend

[jlariza]

Summary

Adds a new VercelBlobStorage backend for VercelBlob, using the official vercel-py SDK.

What's included

• storages/backends/vercel_blob.py — full implementation of the DjangoStorage API:
  • _open, _save, delete, exists, listdir, size, url, get_available_name
  • VercelBlobFile wrapping SpooledTemporaryFile for lazy reads and buffered writes
  • Both "public" and "private" access modes
  • URL construction from the bearer token without a network round-trip, with a head() fallback
  • Paginated listdir using mode="folded" to surface virtual directories
  • Token resolution via Django setting → BLOB_READ_WRITE_TOKEN → VERCEL_BLOB_READ_WRITE_TOKEN
• tests/test_vercel_blob.py — 59 unit tests (fully mocked, no network calls)
• docs/backends/vercel-blob.rst — full documentation covering installation, settings, private-blob proxy pattern, URL behaviour, security notes, and known limitations
• pyproject.toml — new vercel-blob optional dependency group (vercel>=0.5.0)

Notes

• The vercel package requires Python 3.10+. This is documented in the installation note and in the backend's docstring; the rest of django-storages continues to support Python 3.8+.
• Vercel Blob has no presigned/expiring URL support. The docs include a proxy-view pattern for serving private blobs from Django.
• The sync SDK creates a fresh HTTP client per call, so the backend is thread-safe without any additional locking.
• Claude Code was used in the coding process. However, everything was reviewed and corrected by humans when necessary.

Checklist

• Tests pass (pytest tests/test_vercel_blob.py — 59/59)
• No type annotations (matches existing backend style)
• Optional dependency declared in pyproject.toml
• Documentation added and linked from docs/index.rst
• Tested with Vercel directly. It properly worked for both default and staticfiles using this configuration

STORAGES = {
    "default": {
        "BACKEND": "storages.backends.vercel_blob.VercelBlobStorage",
        "OPTIONS": {
            "token": "<token>",
            "location": "media",
        },
    },
    "staticfiles": {
        "BACKEND": "storages.backends.vercel_blob.VercelBlobStorage",
        "OPTIONS": {
            "token": "<token>",
        },
    },
}

[jlariza]

@jschneier please, check. Thank you

[dnwpark]

The only things I'd maybe add are support for random suffixes and cache max age. Not that critical.