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/pull-request/pull-request.component.html

@@ -0,0 +1,50 @@
+<div
+  class="op-pull-request--title"
+  [textContent]="pullRequest.title"
+></div>
+
+<span class="op-pull-request--state" [ngClass]="'op-pull-request--state_' + state">
+  {{ state }} |
+  <a
+    [ngClass]="'op-pull-request--link_' + state"
+    [href]="pullRequest.htmlUrl"
+    target="_blank"
+    rel="noopener noreferrer"
+    [textContent]="pullRequest.repository + '#' + pullRequest.number"
+  ></a>
+</span>
+
+@if (pullRequest.labels && pullRequest.labels.length > 0) {
+  <button (click)="toggleLabels(pullRequest.number)" class="op-pull-request--label_icon" title="Show/Hide labels">
+    Labels
+  </button>
+}
+
+<div class="op-pull-request--info_created">
+  {{ text.label_created_by }}
+  <span class="op-pull-request--user">
+    @if (pullRequest.giteaUser) {
+      <span
+        class="op-pull-request--username"
+        [textContent]="pullRequest.giteaUser.login"
+      ></span>
+    }
+  </span>
+  <span class="op-pull-request--date">
+    {{ text.label_last_updated_on }}
+    <op-date-time [dateTimeValue]="pullRequest.gitea_updated_at" />
+  </span>.
+</div>
+
+<div
+  class="op-pull-request--labels"
+  [ngClass]="'op-pull-request--labels-' + pullRequest.number"
+  style="display: none;">
+  @for (label of pullRequest.labels; track label) {
+    <span>
+      <span class="op-pull-request--label" [ngStyle]="{'background-color': label.color}">
+        {{ label.name }}
+      </span>
+    </span>
+  }
+</div>