fix(import): add @Schema annotations and fix IOException test coverage

- Add @Schema(requiredMode = REQUIRED) to SkippedFile and ImportStatus
  record components so TypeScript codegen produces non-optional fields
  when generate:api is next run
- Extract openFileStream(File) as package-private method so the
  IOException path can be tested deterministically without relying on
  OS-level file permissions (which are bypassed when running as root)
- Replace assumeTrue-based IOException test with Mockito spy that stubs
  openFileStream — test now runs in CI unconditionally (45 tests, 0 skipped)

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

backend/src/main/java/org/raddatz/familienarchiv/importing/MassImportService.java

@@ -2,6 +2,7 @@ package org.raddatz.familienarchiv.importing;
 
 import com.fasterxml.jackson.annotation.JsonIgnore;
 import com.fasterxml.jackson.annotation.JsonProperty;
+import io.swagger.v3.oas.annotations.media.Schema;
 import lombok.RequiredArgsConstructor;
 import lombok.extern.slf4j.Slf4j;
 import org.apache.poi.ss.usermodel.*;
@@ -55,9 +56,20 @@ public class MassImportService {
 
     public enum State { IDLE, RUNNING, DONE, FAILED }
 
-    public record SkippedFile(String filename, String reason) {}
+    public record SkippedFile(
+            @Schema(requiredMode = Schema.RequiredMode.REQUIRED) String filename,
+            @Schema(requiredMode = Schema.RequiredMode.REQUIRED) String reason
+    ) {}
 
-    public record ImportStatus(State state, String statusCode, @JsonIgnore String message, int processed, List<SkippedFile> skippedFiles, LocalDateTime startedAt) {
+    public record ImportStatus(
+            @Schema(requiredMode = Schema.RequiredMode.REQUIRED) State state,
+            @Schema(requiredMode = Schema.RequiredMode.REQUIRED) String statusCode,
+            @JsonIgnore String message,
+            @Schema(requiredMode = Schema.RequiredMode.REQUIRED) int processed,
+            @Schema(requiredMode = Schema.RequiredMode.REQUIRED) List<SkippedFile> skippedFiles,
+            LocalDateTime startedAt
+    ) {
+        @Schema(requiredMode = Schema.RequiredMode.REQUIRED)
         @JsonProperty("skipped")
         public int skipped() { return skippedFiles.size(); }
     }
@@ -300,8 +312,12 @@ public class MassImportService {
         return new ProcessResult(processed, skippedFiles);
     }
 
+    InputStream openFileStream(File file) throws IOException {
+        return new FileInputStream(file);
+    }
+
     private boolean isPdfMagicBytes(File file) throws IOException {
-        try (InputStream is = new FileInputStream(file)) {
+        try (InputStream is = openFileStream(file)) {
             byte[] header = is.readNBytes(4);
             return header.length == 4
                     && header[0] == 0x25  // %