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

lib/open_project/gitea_integration/hook_handler.rb

@@ -0,0 +1,87 @@
+#-- copyright
+# OpenProject is an open source project management software.
+# Copyright (C) the OpenProject GmbH
+#
+# This program is free software; you can redistribute it and/or
+# modify it under the terms of the GNU General Public License version 3.
+#
+# OpenProject is a fork of ChiliProject, which is a fork of Redmine. The copyright follows:
+# Copyright (C) 2006-2013 Jean-Philippe Lang
+# Copyright (C) 2010-2013 the ChiliProject Team
+#
+# This program is free software; you can redistribute it and/or
+# modify it under the terms of the GNU General Public License
+# as published by the Free Software Foundation; either version 2
+# of the License, or (at your option) any later version.
+#
+# This program is distributed in the hope that it will be useful,
+# but WITHOUT ANY WARRANTY; without even the implied warranty of
+# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+# GNU General Public License for more details.
+#
+# You should have received a copy of the GNU General Public License
+# along with this program; if not, write to the Free Software
+# Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA  02110-1301, USA.
+#
+# See COPYRIGHT and LICENSE files for more details.
+#++
+
+module OpenProject::GiteaIntegration
+  class HookHandler
+    # Gitea webhook events we handle.
+    KNOWN_EVENTS = %w[
+      pull_request
+      issue_comment
+    ].freeze
+
+    def process(_hook, request, params, user)
+      event_type = request.env["HTTP_X_GITEA_EVENT"].to_s.downcase.tr(" ", "_")
+
+      Rails.logger.debug { "Received Gitea webhook #{event_type}" }
+
+      return 404 unless KNOWN_EVENTS.include?(event_type)
+      return 403 unless authorized?(request, user)
+
+      notify(params, user, event_type)
+      200
+    end
+
+    private
+
+    def authorized?(request, user)
+      valid_signature?(request) &&
+        user.present? &&
+        (configured_user_id.blank? || user.id == configured_user_id)
+    end
+
+    def notify(params, user, event_type)
+      payload = params[:payload]
+                .permit!
+                .to_h
+                .merge("open_project_user_id" => user.id,
+                       "gitea_event" => event_type)
+
+      OpenProject::Notifications.send(:"gitea.#{event_type}", payload)
+    end
+
+    # Gitea uses HMAC-SHA256, same as GitHub (X-Gitea-Signature header).
+    def valid_signature?(request)
+      secret = plugin_settings[:webhook_secret].presence
+      return true if secret.blank?
+
+      signature_header = request.env["HTTP_X_GITEA_SIGNATURE"]
+      return false if signature_header.blank?
+
+      expected = "sha256=#{OpenSSL::HMAC.hexdigest("SHA256", secret, request.raw_post)}"
+      ActiveSupport::SecurityUtils.secure_compare(expected, signature_header)
+    end
+
+    def configured_user_id
+      plugin_settings[:gitea_user_id].presence&.to_i
+    end
+
+    def plugin_settings
+      Hash(Setting.plugin_openproject_gitea_integration).with_indifferent_access
+    end
+  end
+end