Support custom ID types (string/number) for improved compatibility

- Replace hardcoded payload-types imports with generic BaseEmailDocument interface
- Update sendEmail and sendEmailTask to work with both string and number IDs
- Refactor MailingService to use generic document types instead of specific ones
- Add BaseEmailDocument and BaseEmailTemplateDocument interfaces supporting id: string | number
- Export BaseEmailDocument for users to extend with their custom fields
- Fix TypeScript compilation error in template subject handling
- Add CUSTOM-TYPES.md documentation for users with different ID types

Fixes compatibility issue where plugin required number IDs but user projects used string IDs.

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

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

CUSTOM-TYPES.md

@@ -0,0 +1,88 @@
+# Using Custom ID Types
+
+The mailing plugin now supports both `string` and `number` ID types. By default, it works with the generic `BaseEmailDocument` interface, but you can provide your own types for full type safety.
+
+## Usage with Your Generated Types
+
+When you have your own generated Payload types (e.g., from `payload generate:types`), you can use them with the mailing plugin:
+
+```typescript
+import { sendEmail, BaseEmailDocument } from '@xtr-dev/payload-mailing'
+import { Email } from './payload-types' // Your generated types
+
+// Option 1: Use your specific Email type
+const email = await sendEmail<Email>(payload, {
+  template: {
+    slug: 'welcome',
+    variables: { name: 'John' }
+  },
+  data: {
+    to: 'user@example.com',
+    // All your custom fields are now type-safe
+  }
+})
+
+// Option 2: Extend BaseEmailDocument for custom fields
+interface MyEmail extends BaseEmailDocument {
+  customField: string
+  anotherField?: number
+}
+
+const customEmail = await sendEmail<MyEmail>(payload, {
+  data: {
+    to: 'user@example.com',
+    subject: 'Hello',
+    html: '<p>Hello World</p>',
+    customField: 'my value', // Type-safe!
+  }
+})
+```
+
+## ID Type Compatibility
+
+The plugin works with both:
+- **String IDs**: `id: string`
+- **Number IDs**: `id: number`
+
+Your Payload configuration determines which type is used. The plugin automatically adapts to your setup.
+
+## Type Definitions
+
+The base interfaces provided by the plugin:
+
+```typescript
+interface BaseEmailDocument {
+  id: string | number
+  template?: any
+  to: string[]
+  cc?: string[]
+  bcc?: string[]
+  from?: string
+  replyTo?: string
+  subject: string
+  html: string
+  text?: string
+  variables?: Record<string, any>
+  scheduledAt?: string
+  sentAt?: string
+  status?: 'pending' | 'processing' | 'sent' | 'failed'
+  attempts?: number
+  lastAttemptAt?: string
+  error?: string
+  priority?: number
+  createdAt?: string
+  updatedAt?: string
+}
+
+interface BaseEmailTemplateDocument {
+  id: string | number
+  name: string
+  slug: string
+  subject?: string
+  content?: any
+  createdAt?: string
+  updatedAt?: string
+}
+```
+
+These provide a foundation that works with any ID type while maintaining type safety for the core email functionality.