fix(auth): scope media ACL checks to the namespace

Media files have no per-file ACLs; permissions must be evaluated against
the namespace they live in. Several call sites passed the raw media ID
to

fix(auth): scope media ACL checks to the namespace

Media files have no per-file ACLs; permissions must be evaluated against
the namespace they live in. Several call sites passed the raw media ID
to auth_quickaclcheck(), so a page-intended exact-ID rule (e.g. on
wiki:secret.png) could silently apply to a media file sharing that ID.

Introduce mediaAclPath() that builds the correct namespace wildcard
path (handling root-namespace media) and route all media-related ACL
checks through it. Also normalize the lone `:X` sentinel variant in
fetch.functions.php to the standard `:*` form.

fixes: #4647

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