feat(backends): Pool-Modell — Backend = Pool, N Server pro Backend

Migration 0016: backend_servers (id, backend_id, name, address, port,
weight, backup, active) + backends.lb_algorithm. Daten-Migration kopiert
bestehende backends.address/port als ersten Server, dann DROP COLUMN.

HAProxy-Renderer: rendert pro Backend einen Block mit `balance <algo>`
+ N `server`-Zeilen (weight, backup-Flag, optional check inter 5s).
LB-Algorithmen: roundrobin / leastconn / source.

REST: /backends/:id/servers (GET/POST), /backend-servers/:id (PUT/DELETE).
Re-rendert HAProxy nach jeder Server-Mutation.

UI: address/port aus Backend-Form raus, lb_algorithm-Select rein. Server
verwaltet ein expandable Sub-Panel pro Backend-Row (Tabelle + Add/Edit/
Delete-Modal). Domain-Attachment-Multi-Select bleibt.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

management-ui/src/components/Layout/Sidebar.tsx

@@ -77,7 +77,7 @@ const NAV: NavSection[] = [
   },
 ]
 
-const VERSION = '1.0.48'
+const VERSION = '1.0.49'
 
 export default function Sidebar({ isOpen, onClose }: SidebarProps) {
   const { t } = useTranslation()