phase4-6: harden help/docs links and auth defaults

Close remaining low-priority review follow-ups by removing hardcoded help URLs, improving help drawer accessibility, raising explicit bcrypt cost, and adding validation/injection coverage; also bound postqueue reads with a timeout to avoid hangs.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Co-authored-by: Cursor <cursoragent@cursor.com>

internal/web/view/templates/help.html

@@ -32,7 +32,7 @@ on the card itself.</p>
   </ul>
   <p class="muted">Topics marked as labels open the same text in the help
   drawer. For installation, backup, and environment variables see the
-  <a href="https://github.com/mixeme/selfpost/blob/main/docs/guide.md">operator
+  <a href="{{.SourceURL}}/blob/main/docs/guide.md">operator
   guide</a> in the repository.</p>
 </div>