You are an expert full-stack developer and software architect. Your objective is to integrate the GoSMS WhatsApp & SMS Notification Gateway API into this codebase.

## PHASE 1: REPOSITORY & NOTIFICATION FEATURE DISCOVERY
Before generating code, first scan the entire repository/codebase and catalog all user-facing notification events:
1. Authentication & Security:
   - Registration OTP, Login OTP, Two-Step Verification (2FA), Password Reset Code, New Device Login Alert.
2. Subscription & Membership Lifecycle:
   - Subscription purchased / activated confirmation (with plan name, amount, expiry date, invoice number).
   - Subscription expiring soon reminders (e.g. 3 or 7 days prior with days remaining).
   - Subscription expired notices.
   - Subscription renewal success.
3. Billing, Invoices & Receipts:
   - Payment confirmation & PDF Tax Invoice generation (locate where PDF invoices or receipts are generated/saved).
   - Wallet top-up / credit notifications.
4. Form Submission & Tracking Tokens:
   - Contact forms, KYC applications, service requests that generate a Token / Tracking ID.
   - Application Approved / Rejected status notifications.
5. E-Commerce & Deliveries:
   - Order confirmation, Out for Delivery (with Delivery PIN/OTP), Order Delivered.

## PHASE 2: GOSMS API SPECIFICATION
- Base URL: https://www.gosms.in/api/v1
- Auth Header: Authorization: Bearer {YOUR_API_KEY}
- Endpoints:
  1. WhatsApp Message / Template / PDF Media Attachment:
     POST https://www.gosms.in/api/v1/whatsapp/send
     Payload format:
     {
       "to": "10_DIGIT_MOBILE",
       "template_slug": "subscription-activated", // e.g. standard-otp, subscription-activated, subscription-expiring-soon, form-submitted, application-approved, payment-receipt, order-confirmed, out-for-delivery
       "variables": { "NAME": "...", "AMOUNT": "...", "EXPIRY_DATE": "...", "TOKEN_NO": "...", "LINK": "...", "BRAND_NAME": "..." },
       "media_url": "https://yourdomain.com/invoices/inv_123.pdf", // Optional: Attach PDF Invoice or document directly to WhatsApp
       "file_name": "Tax_Invoice_2026.pdf"
     }
  2. WhatsApp Priority OTP:
     POST https://www.gosms.in/api/v1/whatsapp/send-otp
     Payload: { "to": "10_DIGIT_MOBILE", "otp": "123456", "action": "two-step verification", "brand": "YourBrand" }
  3. General SMS:
     POST https://www.gosms.in/api/v1/sms/send
     Payload: { "to": "10_DIGIT_MOBILE", "message": "...", "sender_id": "YOUR_VENDOR_ID" }

## PHASE 3: IMPLEMENTATION
1. Create a reusable GoSMS notification service in this project with environment configuration (GOSMS_API_KEY, GOSMS_BASE_URL).
2. Attach listeners/hooks to all identified app events.
3. For invoice payments and subscription activations, ensure the generated PDF invoice URL is passed into media_url so the customer receives the invoice PDF directly on WhatsApp.
4. Provide comprehensive error logging, retry handling, and graceful fallbacks.

import 'dart:convert';
import 'package:http/http.dart' as http;

Future<void> sendSMS() async {
  final url = Uri.parse('https://www.gosms.in/api/v1/sms');
  final headers = {
    'Authorization': 'YOUR_API_KEY_HERE',
    'Content-Type': 'application/json',
  };
  final body = jsonEncode({
    'sender_id': 'YOUR_VENDOR_ID',
    'numbers': '9876543210',
    'rout': 'sms',
    'variables_values': '123456',
  });

  try {
    final response = await http.post(url, headers: headers, body: body);
    if (response.statusCode == 200) {
      print('Response: ${response.body}');
    } else {
      print('Failed: ${response.statusCode}');
    }
  } catch (e) {
    print('Error: $e');
  }
}

import okhttp3.MediaType.Companion.toMediaType
import okhttp3.OkHttpClient
import okhttp3.Request
import okhttp3.RequestBody.Companion.toRequestBody
import java.io.IOException

fun sendSMS() {
    val client = OkHttpClient()
    val mediaType = "application/json; charset=utf-8".toMediaType()
    val json = """
        {
            "sender_id": "YOUR_VENDOR_ID",
            "numbers": "9876543210",
            "rout": "sms",
            "variables_values": "123456"
        }
    """.trimIndent()
    val body = json.toRequestBody(mediaType)
    val request = Request.Builder()
        .url("https://www.gosms.in/api/v1/sms")
        .post(body)
        .addHeader("Authorization", "YOUR_API_KEY_HERE")
        .addHeader("Content-Type", "application/json")
        .build()

    client.newCall(request).enqueue(object : okhttp3.Callback {
        override fun onFailure(call: okhttp3.Call, e: java.io.IOException) {
            e.printStackTrace()
        }

        override fun onResponse(call: okhttp3.Call, response: okhttp3.Response) {
            response.use {
                if (!response.isSuccessful) throw java.io.IOException("Unexpected code ${response}")
                println(response.body?.string())
            }
        }
    })
}