feat(filename): support compound last names like de Gruyter

Replace the four fixed regexes with a split-based algorithm:
- first segment = date → last segment = firstName, rest = lastName parts
- last segment = date → second-to-last = firstName, rest = lastName parts

18881025_de_Gruyter_Walter.pdf now correctly yields "Walter de Gruyter".
Simple two-segment names behave identically to before.

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

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

@@ -545,6 +545,18 @@ class DocumentServiceTest {
                 .isEqualTo("Hans Mueller (12.03.1965)");
     }
 
+    @Test
+    void titleFromFilename_compound_lastName_dateFirst() {
+        assertThat(DocumentService.titleFromFilename("18881025_de_Gruyter_Walter.pdf"))
+                .isEqualTo("Walter de Gruyter (25.10.1888)");
+    }
+
+    @Test
+    void titleFromFilename_compound_lastName_dateLast() {
+        assertThat(DocumentService.titleFromFilename("de_Gruyter_Walter_18881025.pdf"))
+                .isEqualTo("Walter de Gruyter (25.10.1888)");
+    }
+
     @Test
     void titleFromFilename_fallsBackToStripExtension() {
         assertThat(DocumentService.titleFromFilename("scan_001.pdf")).isEqualTo("scan_001");