fix: Allow optional email values in custom fields

lib/membership/email.ex

@@ -8,10 +8,11 @@ defmodule Mv.Membership.Email do
   addresses according to a standard regex pattern.
 
   ## Validation Rules
-  - Minimum length: 5 characters
+  - **Optional**: `nil` and empty strings are allowed (custom fields are optional)
+  - Minimum length: 5 characters (for non-empty values)
   - Maximum length: 254 characters (RFC 5321 maximum)
   - Pattern: Standard email format (username@domain.tld)
-  - Automatic trimming of leading/trailing whitespace
+  - Automatic trimming of leading/trailing whitespace (empty strings become `nil`)
 
   ## Usage
   This type is used in the CustomFieldValue union type for custom fields with
@@ -46,11 +47,18 @@ defmodule Mv.Membership.Email do
       max_length: @max_length
     ]
 
+  @impl true
+  def cast_input(nil, _), do: {:ok, nil}
+
   @impl true
   def cast_input(value, _) when is_binary(value) do
     value = String.trim(value)
 
     cond do
+      # Empty string after trim becomes nil (optional field)
+      value == "" ->
+        {:ok, nil}
+
       String.length(value) < @min_length ->
         :error