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

Gemfile

@@ -0,0 +1,14 @@
+# frozen_string_literal: true
+
+source "https://rubygems.org"
+
+gemspec
+
+# Point at a local OpenProject checkout for development, or use a released gem.
+# Uncomment and adjust the path as needed:
+#
+#   gem "openproject", path: "../../"
+#
+# Or pin to a specific released version:
+#
+#   gem "openproject", "~> 14.0"