feat(documents): add DocumentDensityResult and MonthBucket records (#385)

Response shape for the upcoming GET /api/documents/density endpoint.
minDate and maxDate are nullable (null on empty archive); buckets is always
present.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

backend/src/main/java/org/raddatz/familienarchiv/document/DocumentDensityResult.java

@@ -0,0 +1,13 @@
+package org.raddatz.familienarchiv.document;
+
+import io.swagger.v3.oas.annotations.media.Schema;
+
+import java.time.LocalDate;
+import java.util.List;
+
+public record DocumentDensityResult(
+        @Schema(requiredMode = Schema.RequiredMode.REQUIRED)
+        List<MonthBucket> buckets,
+        LocalDate minDate,
+        LocalDate maxDate
+) {}

backend/src/main/java/org/raddatz/familienarchiv/document/MonthBucket.java

@@ -0,0 +1,10 @@
+package org.raddatz.familienarchiv.document;
+
+import io.swagger.v3.oas.annotations.media.Schema;
+
+public record MonthBucket(
+        @Schema(requiredMode = Schema.RequiredMode.REQUIRED, example = "1915-08")
+        String month,
+        @Schema(requiredMode = Schema.RequiredMode.REQUIRED)
+        int count
+) {}