Replace GitLab integration with Gitea in issue reporting

Migrated issue reporting functionality from GitLab to Gitea. Updated environment variables, dependencies, and configuration references to support Gitea. Introduced `GiteaClient` for communication with the Gitea API and refactored related code and tests to align with the new integration.

docs/architecture.md

@@ -52,7 +52,6 @@ infrastructure.
 | JDBC driver | `com.mysql:mysql-connector-j` | 9.7.0 |
 | DB migrations | `org.flywaydb:flyway-core` / `flyway-mysql` | 11.20.3 |
 | Error tracking | `io.sentry:sentry` | 8.41.0 |
-| GitLab integration | `org.gitlab4j:gitlab4j-api` | 6.2.0 |
 | HTTP client | `com.squareup.okhttp3:okhttp-jvm` | 5.3.2 |
 | JSON | `org.json:json` + `com.google.code.gson:gson` | — |
 | Classpath scanning | `org.reflections:reflections` | 0.10.2 |