feat: Add H2 in-memory database dependency for fallback scenarios.

backend/pom.xml

@@ -43,6 +43,13 @@
             <groupId>org.springframework.security</groupId>
             <artifactId>spring-security-crypto</artifactId>
         </dependency>
+
+        <!-- H2 in-memory DB (fallback когда нет настроенных тенантов) -->
+        <dependency>
+            <groupId>com.h2database</groupId>
+            <artifactId>h2</artifactId>
+            <scope>runtime</scope>
+        </dependency>
     </dependencies>
 
     <build>