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/archive.html

@@ -0,0 +1,9 @@
+{{define "content"}}
+<h1>Archive</h1>
+<div class="error-box" id="error" role="alert"></div>
+<input type="search" id="archive-search" class="mt"
+  placeholder="Search archived tasks…" aria-label="Search archived tasks"
+  style="max-width:440px; width:100%">
+<p class="muted" style="margin-top:6px">Archived tasks you can access.</p>
+<div class="grid mt" id="archive-list"></div>
+{{end}}