feat: add dashboard rollup helper for portfolio category summaries

src/main/java/io/ventureplatform/util/InvestmentRollup.java

@@ -0,0 +1,85 @@
+package io.ventureplatform.util;
+
+import java.util.Collections;
+import java.util.List;
+import java.util.Map;
+import java.util.stream.Collectors;
+
+/**
+ * Aggregates investment data across portfolio companies
+ * for dashboard rollups.
+ *
+ * <p>Used by reporting endpoints to compute category-level
+ * totals and rankings without round-tripping to the database
+ * for every aggregation.
+ */
+public final class InvestmentRollup {
+
+  private InvestmentRollup() {
+    // utility class
+  }
+
+  /**
+   * Sum the total investment amount for portfolio companies
+   * matching the given category. Returns 0 if no matches.
+   */
+  public static long totalInvestmentByCategory(
+      final List<Map<String, Object>> companies,
+      final String category) {
+    long total = 0;
+    for (Map<String, Object> company : companies) {
+      if (category.equals(company.get("category"))) {
+        total += ((Number) company.get("amount")).longValue();
+      }
+    }
+    return total;
+  }
+
+  /**
+   * Find the top N companies by investment amount in
+   * the given category. If topN is not positive, defaults
+   * to a sensible upper bound.
+   */
+  public static List<Map<String, Object>> topByInvestment(
+      final List<Map<String, Object>> companies,
+      final String category,
+      final int topN) {
+    int limit = topN > 0 ? topN : 100;
+
+    return companies.stream()
+        .filter(c -> category.equals(c.get("category")))
+        .sorted((a, b) -> Long.compare(
+            ((Number) b.get("amount")).longValue(),
+            ((Number) a.get("amount")).longValue()))
+        .limit(limit)
+        .toList();
+  }
+
+  /**
+   * A parameterized SQL fragment: the clause text contains
+   * {@code ?} placeholders and {@code params} holds the
+   * corresponding bind values.
+   */
+  public record CategoryFilter(
+      String clause, List<String> params) { }
+
+  /**
+   * Build a {@code category IN (?, ?, …)} clause with
+   * parameter placeholders for safe use with
+   * {@code PreparedStatement}. Returns {@code "1=0"} with
+   * an empty param list when categories is empty.
+   */
+  public static CategoryFilter buildCategoryFilter(
+      final List<String> categories) {
+    if (categories == null || categories.isEmpty()) {
+      return new CategoryFilter(
+          "1=0", Collections.emptyList());
+    }
+    String placeholders = categories.stream()
+        .map(c -> "?")
+        .collect(Collectors.joining(", "));
+    String clause = "category IN (" + placeholders + ")";
+    return new CategoryFilter(
+        clause, List.copyOf(categories));
+  }
+}