Add OpenProject Gitea integration plugin

Implements a Gitea integration for OpenProject following the same
patterns as the bundled GitHub and GitLab integrations.

- Webhook endpoint at /webhooks/gitea with HMAC-SHA256 verification
- Handles pull_request and issue_comment events
- Links Gitea PRs to work packages via OP# references or URLs
- Posts journal comments on WPs for PR lifecycle events
- Gitea tab on the work package split view
- V3 API endpoint: GET /api/v3/work_packages/:id/gitea_pull_requests
- Admin settings UI for bot user and webhook secret
- Nightly cron job to clean up unlinked PR records

frontend/module/tab-prs/tab-prs.template.html

@@ -0,0 +1,10 @@
+@if (pullRequests.length === 0) {
+  <p
+    class="op-tab-content--text-box"
+    [innerHTML]="getEmptyText()"
+  ></p>
+}
+
+@for (pullRequest of pullRequests; track pullRequest.id || pullRequest.htmlUrl) {
+  <gitea-pull-request [pullRequest]="pullRequest" />
+}