implement Plunk email driver with comprehensive tests

- Add plunkEmail driver following IEmailDriver interface
- Support single and multiple recipients (up to 5 max per Plunk API)
- Handle both string and { text, html } body formats
- Include sender customization (from, name, reply)
- Add recipient validation and error handling
- Create comprehensive unit tests with 6 test cases
- Export plunkEmail from core drivers index
- Update task list with implementation progress

app/src/core/drivers/email/plunk.ts

@@ -0,0 +1,94 @@
+import type { IEmailDriver } from "./index";
+
+export type PlunkEmailOptions = {
+	apiKey: string;
+	host?: string;
+	from?: string;
+};
+
+export type PlunkEmailSendOptions = {
+	to?: string | string[];
+	subscribed?: boolean;
+	name?: string;
+	from?: string;
+	reply?: string;
+	headers?: Record<string, string>;
+};
+
+export type PlunkEmailResponse = {
+	success: boolean;
+	emails: Array<{
+		contact: {
+			id: string;
+			email: string;
+		};
+		email: string;
+	}>;
+	timestamp: string;
+};
+
+export const plunkEmail = (
+	config: PlunkEmailOptions,
+): IEmailDriver<PlunkEmailResponse, PlunkEmailSendOptions> => {
+	const host = config.host ?? "https://api.useplunk.com/v1/send";
+	const from = config.from;
+
+	return {
+		send: async (
+			to: string,
+			subject: string,
+			body: string | { text: string; html: string },
+			options?: PlunkEmailSendOptions,
+		) => {
+			// Determine recipients - options.to takes precedence if provided
+			const recipients = options?.to ?? to;
+
+			// Validate recipient count (Plunk max is 5)
+			const recipientArray = Array.isArray(recipients)
+				? recipients
+				: [recipients];
+			if (recipientArray.length > 5) {
+				throw new Error(
+					"Plunk supports a maximum of 5 recipients per email",
+				);
+			}
+
+			// Build base payload
+			const payload: any = {
+				to: recipients,
+				subject,
+			};
+
+			// Handle body - Plunk only accepts a single body field
+			if (typeof body === "string") {
+				payload.body = body;
+			} else {
+				// When both text and html are provided, send html (degrades gracefully)
+				payload.body = body.html;
+			}
+
+			// Add optional fields from config
+			if (from) {
+				payload.from = from;
+			}
+
+			// Merge with additional options (excluding 'to' since we already handled it)
+			const { to: _, ...restOptions } = options ?? {};
+
+			const res = await fetch(host, {
+				method: "POST",
+				headers: {
+					"Content-Type": "application/json",
+					Authorization: `Bearer ${config.apiKey}`,
+				},
+				body: JSON.stringify({ ...payload, ...restOptions }),
+			});
+
+			if (!res.ok) {
+				throw new Error(`Plunk API error: ${await res.text()}`);
+			}
+
+			return (await res.json()) as PlunkEmailResponse;
+		},
+	};
+};