feat(portal): one-click "View client portal" preview from the project page

Adds a "View client portal" button on the project detail page that opens the
client portal scoped to that project — no CLI. GET /projects/{id}/portal-preview
auto-provisions a client + access token for the project (provision_preview_session)
and seals a portal session cookie, then redirects to /portal.

- Reuses the project's linked client if it has one; otherwise creates/reuses a
  per-project 'preview-<id>' client. Only sets project.client_id when unset, so it
  never clobbers a real client link. Idempotent — repeat clicks reuse the same
  client/token.
- Lives under /projects (not /portal), so a future public proxy exposing only
  /portal/* won't expose this operator shortcut.

Verified: provisioning (unlinked creates+links, idempotent, linked-no-clobber) 7/7.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

templates/projects/detail.html

@@ -4,7 +4,7 @@
 
 {% block content %}
 <!-- Breadcrumb Navigation -->
-<div class="mb-6">
+<div class="mb-6 flex items-center justify-between gap-3">
     <nav class="flex items-center space-x-2 text-sm">
         <a href="/projects" class="text-seismo-orange hover:text-seismo-navy flex items-center">
             <svg class="w-4 h-4 mr-1" fill="none" stroke="currentColor" viewBox="0 0 24 24">
@@ -17,6 +17,17 @@
         </svg>
         <span class="text-gray-900 dark:text-white font-medium" id="project-name-breadcrumb">Project</span>
     </nav>
+
+    <!-- Preview the client portal for this project (opens scoped, read-only) -->
+    <a href="/projects/{{ project_id }}/portal-preview" target="_blank" rel="noopener"
+       class="shrink-0 inline-flex items-center gap-1.5 px-3 py-1.5 text-sm rounded-lg border border-seismo-orange/40 bg-seismo-orange/10 text-seismo-orange hover:bg-seismo-orange/20 transition-colors"
+       title="Preview this project's client portal in a new tab">
+        <svg class="w-4 h-4" fill="none" stroke="currentColor" viewBox="0 0 24 24">
+            <path stroke-linecap="round" stroke-linejoin="round" stroke-width="2" d="M15 12a3 3 0 11-6 0 3 3 0 016 0z"></path>
+            <path stroke-linecap="round" stroke-linejoin="round" stroke-width="2" d="M2.458 12C3.732 7.943 7.523 5 12 5c4.478 0 8.268 2.943 9.542 7-1.274 4.057-5.064 7-9.542 7-4.477 0-8.268-2.943-9.542-7z"></path>
+        </svg>
+        View client portal
+    </a>
 </div>
 
 <!-- Header (loads dynamically) -->