fix(inventory): Instant.MIN/MAX, performed_at-Index, batchReference-Naming und -Validierung, Lasttest-Zeiträume

- Instant.MIN/MAX durch separate Repository-Methoden ersetzt (findAllByPerformedAtAfter/Before)
- DB-Index auf performed_at für Zeitraum-Abfragen
- Naming-Konsistenz: findAllByBatchId → findAllByBatchReference im Domain-Repository
- batchReference-Validierung (Blank-Check) mit InvalidBatchReference-Error
- Lasttest: variierende Zeiträume (7–90 Tage) statt statischem 10-Jahres-Fenster

loadtest/src/test/java/de/effigenix/loadtest/scenario/InventoryScenario.java

@@ -106,9 +106,20 @@ public final class InventoryScenario {
     }
 
     public static ChainBuilder listStockMovementsByDateRange() {
-        return exec(
+        return exec(session -> {
+            var rnd = ThreadLocalRandom.current();
+            int startDay = rnd.nextInt(1, 28);
+            int startMonth = rnd.nextInt(1, 13);
+            int durationDays = rnd.nextInt(7, 90);
+            var from = java.time.LocalDate.of(2026, startMonth, startDay)
+                    .atStartOfDay(java.time.ZoneOffset.UTC).toInstant();
+            var to = from.plus(java.time.Duration.ofDays(durationDays));
+            return session
+                    .set("dateFrom", from.toString())
+                    .set("dateTo", to.toString());
+        }).exec(
                 http("Bestandsbewegungen nach Zeitraum")
-                        .get("/api/inventory/stock-movements?from=2020-01-01T00:00:00Z&to=2030-12-31T23:59:59Z")
+                        .get("/api/inventory/stock-movements?from=#{dateFrom}&to=#{dateTo}")
                         .header("Authorization", "Bearer #{accessToken}")
                         .check(status().is(200))
         );