feat: integrate COS and unify user persistence

pom.xml

@@ -18,6 +18,11 @@
     <java.version>1.8</java.version>
   </properties>
   <dependencies>
+    <dependency>
+      <groupId>com.qcloud</groupId>
+      <artifactId>cos_api</artifactId>
+      <version>5.6.227</version>
+    </dependency>
     <dependency>
       <groupId>org.springframework.boot</groupId>
       <artifactId>spring-boot-starter-web</artifactId>
@@ -51,6 +56,11 @@
       <artifactId>spring-boot-starter-test</artifactId>
       <scope>test</scope>
     </dependency>
+    <dependency>
+      <groupId>com.h2database</groupId>
+      <artifactId>h2</artifactId>
+      <scope>test</scope>
+    </dependency>
   </dependencies>
   <build>
     <plugins>