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

@@ -758,6 +758,21 @@ class MassImportServiceTest {
                 .containsExactly(MassImportService.SkipReason.FILE_READ_ERROR);
     }
 
+    // ─── findFileRecursive — symlink escape security regression — do not remove ─
+
+    @Test
+    void findFileRecursive_throwsDomainException_whenSymlinkEscapesImportDir(
+            @TempDir Path importDirPath, @TempDir Path outsideDir) throws Exception {
+        Path outsideFile = outsideDir.resolve("secret.pdf");
+        Files.writeString(outsideFile, "sensitive content");
+        Files.createSymbolicLink(importDirPath.resolve("secret.pdf"), outsideFile);
+
+        ReflectionTestUtils.setField(service, "importDir", importDirPath.toString());
+
+        assertThatThrownBy(() -> ReflectionTestUtils.invokeMethod(service, "findFileRecursive", "secret.pdf"))
+                .isInstanceOf(DomainException.class);
+    }
+
     // ─── readOds — XXE security regression ───────────────────────────────────
 
     // Security regression — do not remove.