refactor(import): make import directory @Value-configurable

The hardcoded `static final String IMPORT_DIR = "/import"` was the only
non-`@Value` configurable input in MassImportService — every column
index next to it is wired through `app.import.col.*`. Lifts the
contract from infrastructure (compose bind mount) into application
config (`app.import.dir`), with `/import` as the default so the existing
bind-mount path keeps working.

Addresses review feedback from Markus and Felix on #526.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

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

@@ -99,7 +99,9 @@ public class MassImportService {
     @Value("${app.import.col.transcription:13}")
     private int colTranscription;
 
-    private static final String IMPORT_DIR = "/import";
+    @Value("${app.import.dir:/import}")
+    private String importDir;
+
     private static final DateTimeFormatter GERMAN_DATE = DateTimeFormatter.ofPattern("d. MMMM yyyy", Locale.GERMAN);
 
     // ODS XML namespaces
@@ -129,7 +131,7 @@ public class MassImportService {
     }
 
     private File findSpreadsheetFile() throws IOException {
-        try (Stream<Path> files = Files.list(Paths.get(IMPORT_DIR))) {
+        try (Stream<Path> files = Files.list(Paths.get(importDir))) {
             return files
                     .filter(p -> {
                         String name = p.toString().toLowerCase();
@@ -137,7 +139,7 @@ public class MassImportService {
                     })
                     .findFirst()
                     .orElseThrow(() -> new RuntimeException(
-                            "Keine Tabellendatei (.ods/.xlsx/.xls) in " + IMPORT_DIR + " gefunden!"))
+                            "Keine Tabellendatei (.ods/.xlsx/.xls) in " + importDir + " gefunden!"))
                     .toFile();
         }
     }
@@ -378,7 +380,7 @@ public class MassImportService {
     }
 
     private Optional<File> findFileRecursive(String filename) {
-        try (Stream<Path> walk = Files.walk(Paths.get(IMPORT_DIR))) {
+        try (Stream<Path> walk = Files.walk(Paths.get(importDir))) {
             return walk.filter(p -> !Files.isDirectory(p))
                     .filter(p -> p.getFileName().toString().equals(filename))
                     .map(Path::toFile)

backend/src/test/java/org/raddatz/familienarchiv/importing/MassImportServiceTest.java

@@ -50,6 +50,7 @@ class MassImportServiceTest {
     void setUp() {
         service = new MassImportService(documentService, personService, tagService, s3Client, thumbnailAsyncRunner);
         ReflectionTestUtils.setField(service, "bucketName", "test-bucket");
+        ReflectionTestUtils.setField(service, "importDir", "/import");
         ReflectionTestUtils.setField(service, "colIndex", 0);
         ReflectionTestUtils.setField(service, "colBox", 1);
         ReflectionTestUtils.setField(service, "colFolder", 2);
@@ -79,6 +80,19 @@ class MassImportServiceTest {
         assertThat(service.getStatus().state()).isEqualTo(MassImportService.State.FAILED);
     }
 
+    @Test
+    void runImportAsync_readsFromConfiguredImportDir(@TempDir Path tempDir) {
+        // Empty temp dir → findSpreadsheetFile throws "no spreadsheet" with the
+        // configured path in the message. Proves the field, not a constant,
+        // drives the lookup.
+        ReflectionTestUtils.setField(service, "importDir", tempDir.toString());
+
+        service.runImportAsync();
+
+        assertThat(service.getStatus().state()).isEqualTo(MassImportService.State.FAILED);
+        assertThat(service.getStatus().message()).contains(tempDir.toString());
+    }
+
     @Test
     void runImportAsync_throwsConflict_whenAlreadyRunning() {
         MassImportService.ImportStatus running = new MassImportService.ImportStatus(