feat: add automatic update checker with version injection

- Add internal/updater package (queries Gitea releases API every 6h)

- Inject version at build time via -ldflags (-X main.Version)

- Show update badge in header for admin/owner users

- Show version on system info page

- Add VERSION build arg to Dockerfile

- Update docs (deployment, architecture, admin-guide, contributing, README)

web/templates/system_info.html

@@ -9,6 +9,17 @@
       <div class="card-body">
         {{with .SystemInfo}}
         <div class="datagrid">
+          <div class="datagrid-item">
+            <div class="datagrid-title">Keywarden Version</div>
+            <div class="datagrid-content">
+              <span class="badge bg-blue-lt">{{appVersion}}</span>
+              {{if updateAvailable}}
+              <a href="{{releaseURL}}" target="_blank" rel="noopener noreferrer" class="badge bg-yellow-lt ms-1" title="Update verfügbar">
+                <i class="ti ti-download"></i> {{latestVersion}} verfügbar
+              </a>
+              {{end}}
+            </div>
+          </div>
           <div class="datagrid-item">
             <div class="datagrid-title">Runtime Environment</div>
             <div class="datagrid-content">