Prepare SSE mechanism

pom.xml

@@ -8,6 +8,19 @@
   <version>1.0-SNAPSHOT</version>
   <packaging>pom</packaging>
 
+  <!-- CB: Temporary add my repository, while waiting for SSE Java server module author to incorporate my PR or for me to fork it --> 
+  <repositories>
+    <repository>
+      <id>republicate.com</id>
+      <url>https://republicate.com/maven2</url>
+      <releases>
+        <enabled>true</enabled>
+        <updatePolicy>always</updatePolicy>
+        <checksumPolicy>fail</checksumPolicy>
+      </releases>
+    </repository>
+  </repositories>
+
   <properties>
     <project.build.sourceEncoding>UTF-8</project.build.sourceEncoding>
     <maven.compiler.source>11</maven.compiler.source>

webapp/pom.xml

@@ -225,6 +225,12 @@
             <artifactId>pdfbox</artifactId>
             <version>2.0.28</version>
         </dependency>
+        <!-- server-side events -->
+        <dependency>
+            <groupId>com.republicate</groupId>
+            <artifactId>jeasse-servlet3</artifactId>
+            <version>1.2</version>
+        </dependency>
         <!-- test emails -->
         <dependency>
             <groupId>com.icegreen</groupId>

webapp/src/main/kotlin/org/jeudego/pairgoth/web/sse/SSEServlet.kt

@@ -0,0 +1,21 @@
+package org.jeudego.pairgoth.web.sse
+
+import info.macias.sse.EventBroadcast
+import info.macias.sse.servlet3.ServletEventTarget
+import org.slf4j.LoggerFactory
+import javax.servlet.http.HttpServlet
+import javax.servlet.http.HttpServletRequest
+import javax.servlet.http.HttpServletResponse
+
+
+class SSEServlet: HttpServlet() {
+    companion object {
+        private val logger = LoggerFactory.getLogger("sse")
+    }
+    private val broadcast = EventBroadcast()
+
+    override fun doGet(req: HttpServletRequest, resp: HttpServletResponse?) {
+        logger.trace("<< new channel")
+        broadcast.addSubscriber(ServletEventTarget(req), req.getHeader("Last-Event-Id"))
+    }
+}

webapp/src/main/webapp/WEB-INF/web.xml

@@ -31,12 +31,22 @@
     <servlet-name>api</servlet-name>
     <servlet-class>org.jeudego.pairgoth.web.ApiServlet</servlet-class>
   </servlet>
+  <servlet>
+    <servlet-name>sse</servlet-name>
+    <servlet-class>org.jeudego.pairgoth.web.sse.SSEServlet</servlet-class>
+    <load-on-startup>1</load-on-startup>
+    <async-supported>true</async-supported>
+  </servlet>
 
   <!-- servlet mappings -->
   <servlet-mapping>
     <servlet-name>api</servlet-name>
     <url-pattern>/api/*</url-pattern>
   </servlet-mapping>
+  <servlet-mapping>
+    <servlet-name>sse</servlet-name>
+    <url-pattern>/events/*</url-pattern>
+  </servlet-mapping>
 
   <!-- context params -->
   <context-param>