feat(stats): add GET /api/stats endpoint returning totalPersons + totalDocuments

New StatsController + StatsDTO; no WRITE_ALL required (read-only aggregates).

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

backend/src/main/java/org/raddatz/familienarchiv/controller/StatsController.java

@@ -0,0 +1,25 @@
+package org.raddatz.familienarchiv.controller;
+
+import org.raddatz.familienarchiv.dto.StatsDTO;
+import org.raddatz.familienarchiv.repository.DocumentRepository;
+import org.raddatz.familienarchiv.repository.PersonRepository;
+import org.springframework.http.ResponseEntity;
+import org.springframework.web.bind.annotation.GetMapping;
+import org.springframework.web.bind.annotation.RequestMapping;
+import org.springframework.web.bind.annotation.RestController;
+
+import lombok.RequiredArgsConstructor;
+
+@RestController
+@RequestMapping("/api/stats")
+@RequiredArgsConstructor
+public class StatsController {
+
+    private final PersonRepository personRepository;
+    private final DocumentRepository documentRepository;
+
+    @GetMapping
+    public ResponseEntity<StatsDTO> getStats() {
+        return ResponseEntity.ok(new StatsDTO(personRepository.count(), documentRepository.count()));
+    }
+}

backend/src/main/java/org/raddatz/familienarchiv/dto/StatsDTO.java

@@ -0,0 +1,7 @@
+package org.raddatz.familienarchiv.dto;
+
+/**
+ * Aggregate counts for the dashboard/persons stats bar.
+ */
+public record StatsDTO(long totalPersons, long totalDocuments) {
+}