refactor: update tenant data source configuration and routing logic.

backend/src/main/java/com/magistr/app/config/tenant/TenantDataSourceConfig.java

@@ -103,6 +103,7 @@ public class TenantDataSourceConfig implements WebMvcConfigurer {
 
         HibernateJpaVendorAdapter vendorAdapter = new HibernateJpaVendorAdapter();
         vendorAdapter.setGenerateDdl(true);
+        vendorAdapter.setDatabasePlatform("org.hibernate.dialect.PostgreSQLDialect");
         em.setJpaVendorAdapter(vendorAdapter);
 
         Map<String, Object> props = new HashMap<>();

backend/src/main/java/com/magistr/app/config/tenant/TenantRoutingDataSource.java

@@ -33,7 +33,16 @@ public class TenantRoutingDataSource extends AbstractRoutingDataSource {
     @Override
     protected Object determineCurrentLookupKey() {
         String tenant = TenantContext.getCurrentTenant();
-        return tenant != null ? tenant : "default";
+        if (tenant != null && dataSources.containsKey(tenant)) {
+            return tenant;
+        }
+        // Fallback: если нет контекста (например при инициализации JPA)
+        // или тенант не найден — берём первый доступный
+        if (!dataSources.isEmpty()) {
+            String fallback = dataSources.keySet().iterator().next().toString();
+            return fallback;
+        }
+        return "default";
     }
 
     /**