Fix variables field type to support all JSON-compatible values

- Replace restrictive Record<string, any> with flexible JSONValue type for variables field
- Add JSONValue type alias that matches Payload's JSON field type specification
- Support string, number, boolean, objects, arrays, null, and undefined for variables
- Update both BaseEmailDocument and QueuedEmail interfaces consistently
- Update documentation to reflect JSONValue support

Fixes type constraint error where customer Email.variables field type
(string | number | boolean | {...} | unknown[] | null | undefined)
was not assignable to Record<string, any>.

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <noreply@anthropic.com>

CUSTOM-TYPES.md

@@ -53,6 +53,9 @@ Your Payload configuration determines which types are used. The plugin automatic
 The base interfaces provided by the plugin:
 
 ```typescript
+// JSON value type that matches Payload's JSON field type
+type JSONValue = string | number | boolean | { [k: string]: unknown } | unknown[] | null | undefined
+
 interface BaseEmailDocument {
   id: string | number
   template?: any
@@ -64,7 +67,7 @@ interface BaseEmailDocument {
   subject: string
   html: string
   text?: string | null
-  variables?: Record<string, any> | null
+  variables?: JSONValue  // Supports any JSON-compatible value
   scheduledAt?: string | null
   sentAt?: string | null
   status?: 'pending' | 'processing' | 'sent' | 'failed' | null