security(import): add canonical path escape guard in findFileRecursive

A symlink placed inside importDir pointing to a file outside it would pass
isValidImportFilename (no forbidden chars in the symlink name) and be found
by Files.walk. Now checks candidate.getCanonicalPath() against
baseDir.getCanonicalPath() — if the resolved path escapes importDir,
throws DomainException.internal and aborts the import. Adds regression
test using @TempDir + Files.createSymbolicLink.

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

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

@@ -490,11 +490,18 @@ public class MassImportService {
     }
 
     private Optional<File> findFileRecursive(String filename) {
-        try (Stream<Path> walk = Files.walk(Paths.get(importDir))) {
-            return walk.filter(p -> !Files.isDirectory(p))
+        File baseDir = new File(importDir);
+        try (Stream<Path> walk = Files.walk(baseDir.toPath())) {
+            Optional<Path> match = walk.filter(p -> !Files.isDirectory(p))
                     .filter(p -> p.getFileName().toString().equals(filename))
-                    .map(Path::toFile)
                     .findFirst();
+            if (match.isEmpty()) return Optional.empty();
+            File candidate = match.get().toFile();
+            String baseDirCanonical = baseDir.getCanonicalPath();
+            if (!candidate.getCanonicalPath().startsWith(baseDirCanonical + File.separator)) {
+                throw DomainException.internal(ErrorCode.INTERNAL_ERROR, "Path escape detected: " + candidate);
+            }
+            return Optional.of(candidate);
         } catch (IOException e) {
             return Optional.empty();
         }