Add Autosend node with Mail and Contact resources

Implement n8n node for Autosend API with support for:
- Mail resource: Send single and bulk emails with template or custom content
- Contact resource: Create/update contacts (upsert) and get contacts by ID or email
- API key authentication
- Declarative routing following n8n best practices
- Full TypeScript support with proper typing
- Passing all linting checks

credentials/AutosendApi.credentials.ts

@@ -0,0 +1,53 @@
+import type {
+	IAuthenticateGeneric,
+	ICredentialTestRequest,
+	ICredentialType,
+	INodeProperties,
+} from 'n8n-workflow';
+
+export class AutosendApi implements ICredentialType {
+	name = 'autosendApi';
+
+	displayName = 'Autosend API';
+
+	documentationUrl = 'https://docs.autosend.com/';
+
+	icon = 'file:../icons/autosend.svg' as const;
+
+	httpRequestNode = {
+		name: 'Autosend',
+		docsUrl: 'https://docs.autosend.com/',
+		apiBaseUrl: 'https://api.autosend.com/',
+	};
+
+	properties: INodeProperties[] = [
+		{
+			displayName: 'API Key',
+			name: 'apiKey',
+			type: 'string',
+			typeOptions: {
+				password: true,
+			},
+			default: '',
+			required: true,
+			description: 'API key from your Autosend account settings',
+		},
+	];
+
+	authenticate: IAuthenticateGeneric = {
+		type: 'generic',
+		properties: {
+			headers: {
+				Authorization: '=Bearer {{$credentials.apiKey}}',
+			},
+		},
+	};
+
+	test: ICredentialTestRequest = {
+		request: {
+			baseURL: 'https://api.autosend.com',
+			url: '/contacts',
+			method: 'GET',
+		},
+	};
+}