linkcheck reports uppercase-scheme URIs as broken

Describe the bug

Sphinx's linkcheck builder uses a case-sensitive regex (([a-z]+:)?//) to detect non-HTTP URI schemes. URIs with uppercase schemes like FTP:// or HTTP:// are not recognized as external links, causing them to be checked as local file paths and reported as [broken].

Per RFC 3986 §3.1, URI schemes are case-insensitive. The code at line 471-473 of sphinx/builders/linkcheck.py intends to return unchecked for non-HTTP schemes (e.g. FTP), but fails for uppercase variants because the regex only matches lowercase.

How to Reproduce

import io
import shutil
from pathlib import Path
from sphinx.application import Sphinx

root = Path("/tmp/sphinx_linkcheck_test")
shutil.rmtree(root, ignore_errors=True)
(root / "_build").mkdir(parents=True)
(root / "conf.py").write_text("")
(root / "index.rst").write_text(
    "Test\n====\n\n`download <FTP://example.test/file>`_\n"
)

app = Sphinx(str(root), str(root), str(root / "_build"), str(root / "_doctree"),
             "linkcheck", status=io.StringIO(), warning=io.StringIO())
app.build(force_all=True)
print((root / "_build" / "output.txt").read_text())

Expected output:

index.rst:4: [unchecked] FTP://example.test/file

Actual output:

index.rst:4: [broken] FTP://example.test/file:

Environment Information

Sphinx: 9.0.4
Python: 3.11.13
Platform: Linux

Sphinx extensions

[]

Additional context

This regression was introduced by PR #7985.

The relevant code path in sphinx/builders/linkcheck.py:

uri_re = re.compile('([a-z]+:)?//')  # line 64 — case-sensitive (https://github.com/sphinx-doc/sphinx/blob/master/sphinx/builders/linkcheck.py#L64)

if not uri.startswith(('http:', 'https:')):
    if uri_re.match(uri): # https://github.com/sphinx-doc/sphinx/blob/master/sphinx/builders/linkcheck.py#L472
        # Non-supported URI schemes (ex. ftp)
        return _Status.UNCHECKED, '', 0    # ← intended for FTP://, but never reached
    # falls through to local file check → reports [broken]

A fix would be making the regex case-insensitive: re.compile('([a-z]+:)?//', re.IGNORECASE) or using [a-zA-Z]+.

Start in sphinx/builders/linkcheck.py around the URI regex at lines 64 and 471-473, then reproduce the issue with the Sphinx application example in the report. Verify that uppercase-scheme URIs such as FTP://example.test/file are treated as external and produce [unchecked] rather than [broken], while existing HTTP and HTTPS handling remains unchanged.