Migrate HTTP server to Javalin and remove legacy handlers

Replaced the existing HttpServer implementation and associated handler classes (`AbstractHandler`, `ActionHandler`, `GuildHandler`, `RootHandler`) with a Javalin-based routing system. Improved modularity by introducing `ActionRoutes` and `GuildRoutes` for route definitions, and centralized JSON mapping using `GsonJsonMapper`. Simplified caching via the new `ResponseCache` utility, updated `WebModule` for cleaner timeout handling, and extended test coverage for API endpoints.

pom.xml

@@ -183,6 +183,12 @@
             <version>5.3.2</version>
         </dependency>
 
+        <dependency>
+            <groupId>io.javalin</groupId>
+            <artifactId>javalin</artifactId>
+            <version>6.7.0</version>
+        </dependency>
+
         <dependency>
             <groupId>org.hibernate.orm</groupId>
             <artifactId>hibernate-core</artifactId>
@@ -219,6 +225,13 @@
             <groupId>org.gitlab4j</groupId>
             <artifactId>gitlab4j-api</artifactId>
             <version>6.2.0</version>
+            <exclusions>
+                <!-- Clashes with the Servlet 5 API used by Javalin's Jetty 11 in the shaded jar. -->
+                <exclusion>
+                    <groupId>jakarta.servlet</groupId>
+                    <artifactId>jakarta.servlet-api</artifactId>
+                </exclusion>
+            </exclusions>
         </dependency>
 
         <dependency>