feat(bank-account): add student implementation template for concurrency assignment

- Provide clean BankAccount skeleton for student implementation
- Define thread-safety responsibilities for getBalance, deposit, withdraw, and transfer
- Add detailed TODO comments clarifying concurrency and deadlock requirements
- Include hints for lock strategy and global ordering for transfer method
- Ensure compatibility with DemoApplication and stress testing infrastructure

src/main/java/dev/banking/model/BankAccount.java

@@ -7,7 +7,11 @@ public class BankAccount {
 
     /*
      * Students may introduce additional fields
-     * if required by their synchronization strategy.
+     * such as:
+     * - Lock / ReentrantLock
+     * - ReadWriteLock
+     * - Object monitor
+     * - etc.
      */
 
     public BankAccount(int accountId, long initialBalance) {
@@ -20,34 +24,55 @@ public class BankAccount {
     }
 
     /*
-     * TODO
-     * Implement a thread-safe balance reader.
+     * TODO:
+     * Return the current balance in a thread-safe way.
+     *
+     * Requirements:
+     * - Must be safe under concurrent reads/writes
+     * - Should not block unnecessarily if using read/write locks
      */
     public long getBalance() {
-        throw new UnsupportedOperationException();
+        throw new UnsupportedOperationException("TODO: implement thread-safe balance read");
     }
 
     /*
-     * TODO
-     * Implement a thread-safe deposit operation.
+     * TODO:
+     * Increase balance atomically.
+     *
+     * Requirements:
+     * - Must not lose updates under concurrency
      */
     public void deposit(long amount) {
-        throw new UnsupportedOperationException();
+        throw new UnsupportedOperationException("TODO: implement thread-safe deposit");
     }
 
     /*
-     * TODO
-     * Implement a thread-safe withdrawal operation.
+     * TODO:
+     * Decrease balance atomically.
+     *
+     * Requirements:
+     * - Must not cause race conditions
+     * - Negative balance handling is NOT required unless you decide
+     *   to extend the system (optional)
      */
     public void withdraw(long amount) {
-        throw new UnsupportedOperationException();
+        throw new UnsupportedOperationException("TODO: implement thread-safe withdraw");
     }
 
     /*
-     * TODO
-     * Implement an atomic and deadlock-free transfer.
+     * TODO:
+     * Transfer money between two accounts atomically.
+     *
+     * IMPORTANT REQUIREMENTS:
+     * - Must be atomic (no partial transfer)
+     * - Must be deadlock-free
+     * - Must protect both source and target accounts
+     *
+     * HINT:
+     * - Consider global lock ordering using accountId
+     * - Or tryLock with retry strategy
      */
     public void transfer(BankAccount target, long amount) {
-        throw new UnsupportedOperationException();
+        throw new UnsupportedOperationException("TODO: implement atomic deadlock-free transfer");
     }
 }