Remove unused hostname_prefix from server types and add duplicate checking

The HostnamePrefix field on ServerType was loaded from YAML but never used —
hostnames are user-provided. This removes the field and adds explicit
duplicate checks (hostname + MAC) with clear per-field error messages in
both the JSON API and web UI, backed by a new GetByHostname store method
with case-insensitive matching.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

internal/model/model.go

@@ -85,13 +85,12 @@ type ActivityEntry struct {
 }
 
 type ServerType struct {
-	Key            string
-	DisplayName    string   `yaml:"display_name"`
-	BootDisk       string   `yaml:"boot_disk"`
-	ManagementNIC  string   `yaml:"management_nic"`
-	GPU            bool     `yaml:"gpu"`
-	HostnamePrefix string   `yaml:"hostname_prefix"`
-	ExpectedNICs   []NICDef `yaml:"expected_nics"`
+	Key           string
+	DisplayName   string   `yaml:"display_name"`
+	BootDisk      string   `yaml:"boot_disk"`
+	ManagementNIC string   `yaml:"management_nic"`
+	GPU           bool     `yaml:"gpu"`
+	ExpectedNICs  []NICDef `yaml:"expected_nics"`
 }
 
 type NICDef struct {