fix(backend): explicit Flyway config and DataInitializer null title fix

Adding explicit spring.flyway.* config (url/user/password) ensures Flyway
creates its own JDBC connection and runs migrations independently of the JPA
datasource initialization order in Spring Boot 4.0.

Fix DataInitializer creating a Document with title=null, which would hit the
NOT NULL constraint in the documents table once the admin user init succeeds.

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

backend/src/main/java/org/raddatz/familienarchiv/config/DataInitializer.java

@@ -152,9 +152,9 @@ public class DataInitializer {
                     .receivers(Set.of(maria))
                     .build());
 
-            // 5. Document with no title — tests fallback to originalFilename
+            // 5. Document with minimal metadata — tests sparse display
             docRepo.save(Document.builder()
-                    .title(null)
+                    .title("Scan ohne Titel")
                     .originalFilename("scan_ohne_titel.pdf")
                     .status(DocumentStatus.UPLOADED)
                     .documentDate(LocalDate.of(1978, 11, 20))