chore: use interfaces for web related services

internal/app/api/v0/handlers/encoding.go

@@ -5,6 +5,9 @@ import (
 	"strings"
 )
 
+// Base64UrlDecode decodes a base64 url encoded string.
+// In comparison to the standard base64 encoding, the url encoding uses - instead of + and _ instead of /
+// as well as . instead of =.
 func Base64UrlDecode(in string) string {
 	in = strings.ReplaceAll(in, "-", "=")
 	in = strings.ReplaceAll(in, "_", "/")