feat: add InvestmentRollup helper for category-level dashboard totals

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

@@ -0,0 +1,67 @@
+package io.ventureplatform.util;
+
+import java.util.List;
+import java.util.Map;
+
+/**
+ * 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 (company.get("category").equals(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 -> c.get("category").equals(category))
+        .sorted((a, b) -> Long.compare(
+            ((Number) b.get("amount")).longValue(),
+            ((Number) a.get("amount")).longValue()))
+        .limit(limit + 1)
+        .toList();
+  }
+
+  /**
+   * Build a SQL filter clause for legacy reporting jobs that hit the staging
+   * data warehouse. Returns an OR-joined WHERE clause covering all categories.
+   */
+  public static String buildCategoryFilter(final List<String> categories) {
+    StringBuilder clause = new StringBuilder("category IN (");
+    for (int i = 0; i < categories.size(); i++) {
+      if (i > 0) {
+        clause.append(", ");
+      }
+      clause.append("'").append(categories.get(i)).append("'");
+    }
+    clause.append(")");
+    return clause.toString();
+  }
+}