fix: match PackageBlob.HashBlake2b definition and migration (go-gitea#7543)

If not a migration will show a warning:

`[W] Table public.package_blob column hash_blake2b db type is VARCHAR(255), struct type is CHAR(128)`

Reviewed-on: https://codeberg.org/forgejo/forgejo/pulls/7543
Reviewed-by: Michael Kriese <michael.kriese@gmx.de>
Reviewed-by: Beowulf <beowulf@beocode.eu>
Co-authored-by: Earl Warren <contact@earl-warren.org>
Co-committed-by: Earl Warren <contact@earl-warren.org>

Author: earl-warren

models/forgejo_migrations/v26.go

@@ -7,8 +7,8 @@ import "xorm.io/xorm"
 
 func AddHashBlake2bToPackageBlob(x *xorm.Engine) error {
 	type PackageBlob struct {
-		ID          int64 `xorm:"pk autoincr"`
-		HashBlake2b string
+		ID          int64  `xorm:"pk autoincr"`
+		HashBlake2b string `xorm:"hash_blake2b char(128) UNIQUE(blake2b) INDEX"`
 	}
 	return x.Sync(&PackageBlob{})
 }