fix(flyway): baseline existing schemas at V4 on first run

Local dev databases that existed before Flyway was introduced have tables
but no flyway_schema_history. Flyway refuses to migrate a non-empty schema
without a history table. baselineOnMigrate=true with baselineVersion=4
stamps those databases as already at V4 without re-running migrations.
Fresh databases (CI) have an empty schema so the baseline is never
triggered and all 4 migrations run normally.

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

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

@@ -21,6 +21,8 @@ public class FlywayConfig {
         Flyway flyway = Flyway.configure()
                 .dataSource(dataSource)
                 .locations("classpath:db/migration")
+                .baselineOnMigrate(true)
+                .baselineVersion("4")
                 .load();
         var result = flyway.migrate();
         log.info("Flyway: {} migration(s) applied.", result.migrationsExecuted);