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>

internal/haproxy/haproxy.cfg.tpl

@@ -83,8 +83,15 @@ frontend internal_stats
 backend api_backend
     server api1 127.0.0.1:9443 check
 
-{{- range .Backends}}
+{{- range $b := .Backends}}
 
-backend eg_backend_{{.ID}}
-    server {{.Name | safeID}} {{.Address}}:{{.Port}}{{if eq .Scheme "https"}} ssl verify none alpn h2,http/1.1{{end}}{{if .HealthCheckPath}} check inter 5s{{end}}
+backend eg_backend_{{$b.ID}}
+    balance {{$b.LBAlgorithm}}
+    {{- if $b.HealthCheckPath}}
+    option httpchk
+    http-check send meth GET uri {{$b.HealthCheckPath}}
+    {{- end}}
+    {{- range $s := $b.Servers}}
+    server {{$s.Name | safeID}} {{$s.Address}}:{{$s.Port}}{{if eq $b.Scheme "https"}} ssl verify none alpn h2,http/1.1{{end}}{{if $b.HealthCheckPath}} check inter 5s{{end}} weight {{$s.Weight}}{{if $s.Backup}} backup{{end}}
+    {{- end}}
 {{- end}}