Add BookSummary record projection (corner-scenario enrichment)

src/main/java/com/ankurm/sdjpa4demo/domain/BookSummary.java

@@ -0,0 +1,7 @@
+package com.ankurm.sdjpa4demo.domain;
+
+import java.math.BigDecimal;
+
+// Projection / DTO value object - only what the caller needs. Spring Data populates
+// this via constructor-matching (a class-based projection), no @Query required.
+public record BookSummary(String title, BigDecimal amount) {}