feat(documents): add delete button to document edit form

- DELETE /api/documents/{id} endpoint (204 No Content, WRITE_ALL required)
- DocumentService.deleteDocument() — throws 404 if not found, cascades
  via DB foreign keys (versions, annotations, comments all ON DELETE CASCADE)
- Delete form action in edit page server: redirects to / on success
- Two-step confirmation in the save bar: first click reveals inline
  "Wirklich löschen?" + confirm/cancel, avoiding native browser dialogs
- i18n key doc_delete_confirm added to de/en/es

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

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

@@ -121,6 +121,32 @@ class DocumentControllerTest {
                 .andExpect(status().isOk());
     }
 
+    // ─── DELETE /api/documents/{id} ──────────────────────────────────────────
+
+    @Test
+    void deleteDocument_returns401_whenUnauthenticated() throws Exception {
+        mockMvc.perform(org.springframework.test.web.servlet.request.MockMvcRequestBuilders
+                        .delete("/api/documents/" + UUID.randomUUID()))
+                .andExpect(status().isUnauthorized());
+    }
+
+    @Test
+    @WithMockUser
+    void deleteDocument_returns403_whenMissingWritePermission() throws Exception {
+        mockMvc.perform(org.springframework.test.web.servlet.request.MockMvcRequestBuilders
+                        .delete("/api/documents/" + UUID.randomUUID()))
+                .andExpect(status().isForbidden());
+    }
+
+    @Test
+    @WithMockUser(authorities = "WRITE_ALL")
+    void deleteDocument_returns204_whenHasWritePermission() throws Exception {
+        UUID id = UUID.randomUUID();
+        mockMvc.perform(org.springframework.test.web.servlet.request.MockMvcRequestBuilders
+                        .delete("/api/documents/" + id))
+                .andExpect(status().isNoContent());
+    }
+
     // ─── POST /api/documents/quick-upload ────────────────────────────────────
 
     @Test

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

@@ -35,6 +35,29 @@ class DocumentServiceTest {
     @Mock AnnotationService annotationService;
     @InjectMocks DocumentService documentService;
 
+    // ─── deleteDocument ───────────────────────────────────────────────────────
+
+    @Test
+    void deleteDocument_deletesById_whenExists() {
+        UUID id = UUID.randomUUID();
+        when(documentRepository.existsById(id)).thenReturn(true);
+
+        documentService.deleteDocument(id);
+
+        verify(documentRepository).deleteById(id);
+    }
+
+    @Test
+    void deleteDocument_throwsNotFound_whenMissing() {
+        UUID id = UUID.randomUUID();
+        when(documentRepository.existsById(id)).thenReturn(false);
+
+        assertThatThrownBy(() -> documentService.deleteDocument(id))
+                .isInstanceOf(DomainException.class)
+                .hasMessageContaining(id.toString());
+        verify(documentRepository, never()).deleteById(any());
+    }
+
     // ─── getDocumentById ──────────────────────────────────────────────────────
 
     @Test