feat: Archive tab — role-scoped list of archived tasks with search

New /archive page (nav link for all roles) backed by GET /api/v1/archive:
- admins see every archived task;
- consultants see archived tasks for customers they're assigned to;
- developers see archived tasks they were assigned to, claimed, or acted on.
Supports full-text ?q= search; cards link to the task detail view.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

web/templates/layout.html

@@ -28,6 +28,7 @@
         {{end}}
         <a href="/messages" {{if eq .Active "messages"}}aria-current="page"{{end}}>Messages</a>
         <a href="/metrics" {{if eq .Active "metrics"}}aria-current="page"{{end}}>Metrics</a>
+        <a href="/archive" {{if eq .Active "archive"}}aria-current="page"{{end}}>Archive</a>
       {{end}}
     </div>
     <div class="right">