Admin: role merge tool + usage list (taxonomy hygiene)

New /Admin/Roles screen lists every role with its shift/job/talent usage and lets
an admin merge a duplicate role into another — reassigns all listings (the Restrict
FKs) plus preferences/alerts/profiles to the target, then deletes the source — or
toggle a role's visibility. Linked from the admin panel nav (🏷️ نقش‌ها). Lets you
clean up dynamic-ingestion sprawl («کمک‌یار»→«کمک بهیار») without DB surgery.

Improvement 7 of the backlog (data).

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

src/JobsMedical.Web/Pages/Shared/_PanelNav.cshtml

@@ -16,6 +16,7 @@
             <a class="@(On("/Admin/Index") ? "active" : null)" asp-page="/Admin/Index">📥 صف آگهی‌ها</a>
             <a class="@(On("/Admin/Ingested") ? "active" : null)" asp-page="/Admin/Ingested">📜 نتایج جمع‌آوری</a>
             <a class="@(On("/Admin/Facilities") ? "active" : null)" asp-page="/Admin/Facilities">🏥 مراکز</a>
+            <a class="@(On("/Admin/Roles") ? "active" : null)" asp-page="/Admin/Roles">🏷️ نقش‌ها</a>
             <a class="@(On("/Admin/Users") ? "active" : null)" asp-page="/Admin/Users">👥 کاربران</a>
             <a class="@(On("/Admin/Reports") ? "active" : null)" asp-page="/Admin/Reports">🛡️ گزارش‌ها</a>
             <a class="@(On("/Admin/Broadcast") ? "active" : null)" asp-page="/Admin/Broadcast">📣 اعلان همگانی</a>