feat(model): introduce transaction hierarchy with deposit, withdraw, and transfer types

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

@@ -0,0 +1,36 @@
+package dev.banking.model;
+
+/**
+ * Represents a deposit operation.
+ */
+public final class DepositTransaction extends Transaction {
+
+    private final int accountId;
+
+    public DepositTransaction(
+            int accountId,
+            int amount
+    ) {
+        super(amount);
+
+        if (accountId < 0) {
+            throw new IllegalArgumentException(
+                    "Invalid account id."
+            );
+        }
+
+        this.accountId = accountId;
+    }
+
+    public int getAccountId() {
+        return accountId;
+    }
+
+    @Override
+    public String toString() {
+        return "DepositTransaction{" +
+                "accountId=" + accountId +
+                ", amount=" + getAmount() +
+                '}';
+    }
+}