Add per-server admin-page links (Dockge, Webmin, Cockpit, etc.)

Each server's detail page gets an "Admin Links" card for bookmarking
that host's own web UIs -- container managers, Webmin, Cockpit, or
anything else reachable by URL -- so there's a quick way to jump
there without hunting down the address each time.

New server_links table (serverId FK, label, url, ON DELETE CASCADE so
removing a server cleans up its links automatically) and three new
routes: POST/PATCH/DELETE /api/servers/:id/links, gated to operator+
like the rest of this page's editing actions; the existing GET
/:id/detail now includes the server's links alongside hardware/DNS
info. URLs are validated to start with http:// or https:// server-side
(rejecting e.g. a javascript: URL that would otherwise render as a
clickable link).

Rendered as a row of pill buttons (label opens the URL in a new tab),
with inline edit/remove controls next to each when the viewer can
edit, and an "Add link" form matching the existing task-form style on
the same page.

Verified against a temp SQLite DB with real migrations: create/list/
update/delete all work, and deleting the parent server cascades to
remove its links rather than leaving them orphaned.

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

README.md

@@ -42,7 +42,10 @@ All modules from the original plan are built:
   its scheduled tasks — live hardware from Proxmox for VM/LXC-backed
   servers, or from the agent's own hardware report for everything else.
   Proxmox-linked servers also get start/stop/restart buttons right on the
-  detail page.
+  detail page. The detail page also has an **Admin Links** section
+  (operator/admin to add/edit/remove) for bookmarking that server's own
+  admin UIs — Dockge, Webmin, Cockpit, Portainer, or anything else reachable
+  by URL.
 - **Tailscale**, **Proxmox**, **Synology**, **Semaphore**, **Gitea**,
   and **Docker** each get their own top-level page (backed by the
   matching integration) instead of living inside a shared Integrations