refactor(banking-demo): introduce DemoApplication orchestrator and add controlled lifecycle management

- Replace DemoFactory with DemoApplication to centralize system setup, execution, and shutdown
- Move ExecutorService and ScheduledExecutorService lifecycle management into application layer
- Ensure proper shutdown sequence for worker and monitoring threads
- Simplify Main class to a minimal entry-point wrapper
- Improve clarity of execution flow: build → run → shutdown
- Prevent resource leakage by explicitly awaiting termination of thread pools

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

@@ -31,7 +31,7 @@ public class BankAccount {
      * TODO
      * Implement a thread-safe deposit operation.
      */
-    public void deposit(int amount) {
+    public void deposit(long amount) {
         throw new UnsupportedOperationException();
     }
 
@@ -39,7 +39,7 @@ public class BankAccount {
      * TODO
      * Implement a thread-safe withdrawal operation.
      */
-    public void withdraw(int amount) {
+    public void withdraw(long amount) {
         throw new UnsupportedOperationException();
     }
 
@@ -47,7 +47,7 @@ public class BankAccount {
      * TODO
      * Implement an atomic and deadlock-free transfer.
      */
-    public void transfer(BankAccount target, int amount) {
+    public void transfer(BankAccount target, long amount) {
         throw new UnsupportedOperationException();
     }
 }