add an async subscriber impl with thread pool

subscriber-notifs-async/problem.md

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+# System Design Problem: Stock Availability Notification Service
+
+## Problem Statement
+
+Design a Low-Level Design (LLD) for a "Notify Me When Available" feature on an e-commerce platform. When a high-demand product (like a PS5 or iPhone) is out of stock, customers can subscribe to receive an alert when it becomes available again.
+
+## 1. Functional Requirements (FR)
+
+-   **Product Availability Monitoring:** The system must track the stock quantity of products.
+-   **User Subscription:**
+    -   Users can subscribe to a specific product to receive availability alerts.
+    -   Users can choose their preferred communication channel (Email, SMS, or Push Notification) at the time of subscription.
+    -   Users should be able to unsubscribe if they lose interest.
+-   **Notification Trigger:**
+    -   When a product's stock status changes from "Out of Stock" (0) to "In Stock" (>0), the system must trigger alerts to all subscribed users.
+-   **Extensibility:**
+    -   The system must support adding new notification channels (e.g., WhatsApp, Discord webhook) in the future with minimal code changes to the core product logic.
+
+## 2. Non-Functional Requirements (NFR)
+
+-   **Decoupling:** The inventory management system (Product logic) should not be tightly coupled with the notification logic (3rd party Email/SMS services).
+-   **Real-time/Near Real-time:** Notifications should be sent reasonably quickly after stock is updated.
+-   **Scalability:** A single popular product might have 100,000+ subscribers. The notification process should not block the inventory update transaction (e.g., the warehouse manager updating stock shouldn't wait for 100k emails to send before seeing a "Success" message).
+
+---
+
+# Scratch
+
+-   clearly observer pattern ( satisifies decoupling requirement )
+-   nfr 2 and 3 need async processing so we can use message queues or threads but let me implement a basic version first
+
+```
+interface Subscribable  (observer)
+    + subscribe(Subscriber)
+    + unsubscribe(Subscriber)
+    + notifySubscribers()
+
+interface Subscriber
+    + onNotify(Product)
+
+Product (subject) implements Subscribable
+    - List<Subscriber> subscribers
+    + subscribe(Subscriber)
+    + unsubscribe(Subscriber)
+    - notifySubscribers()
+    + setStock(int)
+        - if stock changes from 0 to >0
+            - notifySubscribers()
+```
+
+# Approach
+
+-   made observer pattern with string messages and global notif ( no filer for specific ids )
+-   changed to Event class with runtime polymorphism using base pointer trick
+-   used generics to do compile type polymorphism
+-   make product id a generic too and change to use map of id to list of subscribers
+-   move off the actual notifictions to a thread pool to handle async processing
+
+# Async Handling
+
+-   make a thrad pool class