feat(#145): add recent-activity endpoint sorted by updatedAt

Add GET /api/documents/recent-activity?size=N endpoint that returns
the N most recently updated documents sorted by updatedAt DESC.
Includes TDD: failing tests written first, then production code.

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

backend/src/test/java/org/raddatz/familienarchiv/controller/DocumentControllerTest.java

@@ -406,6 +406,27 @@ class DocumentControllerTest {
                 .andExpect(status().isNoContent());
     }
 
+    // ─── GET /api/documents/recent-activity ──────────────────────────────────
+
+    @Test
+    void getRecentActivity_returns401_whenUnauthenticated() throws Exception {
+        mockMvc.perform(get("/api/documents/recent-activity"))
+                .andExpect(status().isUnauthorized());
+    }
+
+    @Test
+    @WithMockUser
+    void getRecentActivity_returnsOkWithDocuments() throws Exception {
+        Document doc1 = Document.builder().id(UUID.randomUUID()).title("Alpha").originalFilename("a.pdf").build();
+        Document doc2 = Document.builder().id(UUID.randomUUID()).title("Beta").originalFilename("b.pdf").build();
+        when(documentService.getRecentActivity(5)).thenReturn(List.of(doc1, doc2));
+
+        mockMvc.perform(get("/api/documents/recent-activity").param("size", "5"))
+                .andExpect(status().isOk())
+                .andExpect(jsonPath("$[0].title").value("Alpha"))
+                .andExpect(jsonPath("$[1].title").value("Beta"));
+    }
+
     // ─── GET /api/documents/{id}/versions ────────────────────────────────────
 
     @Test

backend/src/test/java/org/raddatz/familienarchiv/service/DocumentServiceTest.java

@@ -1213,4 +1213,26 @@ class DocumentServiceTest {
 
         verify(documentRepository).findAll(any(org.springframework.data.jpa.domain.Specification.class), any(Sort.class));
     }
+
+    // ─── getRecentActivity ────────────────────────────────────────────────────
+
+    @Test
+    void getRecentActivity_returnsMostRecentlyUpdatedDocuments() {
+        java.time.LocalDateTime oldest  = java.time.LocalDateTime.of(2024, 1, 1, 0, 0);
+        java.time.LocalDateTime middle  = java.time.LocalDateTime.of(2024, 6, 1, 0, 0);
+        java.time.LocalDateTime newest  = java.time.LocalDateTime.of(2024, 12, 1, 0, 0);
+
+        Document doc1 = Document.builder().id(UUID.randomUUID()).title("Oldest").build();
+        Document doc2 = Document.builder().id(UUID.randomUUID()).title("Middle").build();
+        Document doc3 = Document.builder().id(UUID.randomUUID()).title("Newest").build();
+
+        // findAll(Sort) returns documents already sorted DESC by updatedAt
+        when(documentRepository.findAll(Sort.by(Sort.Direction.DESC, "updatedAt")))
+                .thenReturn(List.of(doc3, doc2, doc1));
+
+        List<Document> result = documentService.getRecentActivity(2);
+
+        assertThat(result).hasSize(2);
+        assertThat(result).containsExactly(doc3, doc2);
+    }
 }