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/api/smoke_test.go

@@ -134,7 +134,6 @@ func mustLoadServerTypes(t *testing.T, dir string) *config.ServerTypeRegistry {
     boot_disk: "/dev/sda"
     management_nic: "eth0"
     gpu: false
-    hostname_prefix: "pve-test"
 `)
 	if err := writeTestFile(path, content); err != nil {
 		t.Fatal(err)
@@ -337,6 +336,82 @@ func TestOperationNotFound(t *testing.T) {
 	resp.Body.Close()
 }
 
+func TestDuplicateHostnameRejected(t *testing.T) {
+	ts := newTestServer(t)
+	defer ts.Close()
+
+	body := `{"hostname":"pve-dup-01","mac":"aa:bb:cc:dd:ee:d1","server_type":"test-type"}`
+	resp, err := http.Post(ts.URL+"/api/hosts", "application/json", bytes.NewBufferString(body))
+	if err != nil {
+		t.Fatal(err)
+	}
+	if resp.StatusCode != http.StatusCreated {
+		t.Fatalf("first create: got %d, want %d", resp.StatusCode, http.StatusCreated)
+	}
+	resp.Body.Close()
+
+	body = `{"hostname":"pve-dup-01","mac":"aa:bb:cc:dd:ee:d2","server_type":"test-type"}`
+	resp, err = http.Post(ts.URL+"/api/hosts", "application/json", bytes.NewBufferString(body))
+	if err != nil {
+		t.Fatal(err)
+	}
+	if resp.StatusCode != http.StatusConflict {
+		t.Fatalf("dup hostname: got %d, want %d", resp.StatusCode, http.StatusConflict)
+	}
+	var errResp map[string]any
+	json.NewDecoder(resp.Body).Decode(&errResp)
+	resp.Body.Close()
+	if msg, _ := errResp["error"].(string); msg != "a host with this hostname already exists" {
+		t.Fatalf("dup hostname error = %q, want specific hostname message", msg)
+	}
+}
+
+func TestDuplicateMACRejected(t *testing.T) {
+	ts := newTestServer(t)
+	defer ts.Close()
+
+	body := `{"hostname":"pve-mac-01","mac":"aa:bb:cc:dd:ee:e1","server_type":"test-type"}`
+	resp, err := http.Post(ts.URL+"/api/hosts", "application/json", bytes.NewBufferString(body))
+	if err != nil {
+		t.Fatal(err)
+	}
+	if resp.StatusCode != http.StatusCreated {
+		t.Fatalf("first create: got %d, want %d", resp.StatusCode, http.StatusCreated)
+	}
+	resp.Body.Close()
+
+	body = `{"hostname":"pve-mac-02","mac":"aa:bb:cc:dd:ee:e1","server_type":"test-type"}`
+	resp, err = http.Post(ts.URL+"/api/hosts", "application/json", bytes.NewBufferString(body))
+	if err != nil {
+		t.Fatal(err)
+	}
+	if resp.StatusCode != http.StatusConflict {
+		t.Fatalf("dup MAC: got %d, want %d", resp.StatusCode, http.StatusConflict)
+	}
+	var errResp map[string]any
+	json.NewDecoder(resp.Body).Decode(&errResp)
+	resp.Body.Close()
+	if msg, _ := errResp["error"].(string); msg != "a host with this MAC address already exists" {
+		t.Fatalf("dup MAC error = %q, want specific MAC message", msg)
+	}
+}
+
+func TestDuplicateHostnameCaseInsensitive(t *testing.T) {
+	ts := newTestServer(t)
+	defer ts.Close()
+
+	body := `{"hostname":"pve-case-01","mac":"aa:bb:cc:dd:ee:c1","server_type":"test-type"}`
+	resp, _ := http.Post(ts.URL+"/api/hosts", "application/json", bytes.NewBufferString(body))
+	resp.Body.Close()
+
+	body = `{"hostname":"PVE-CASE-01","mac":"aa:bb:cc:dd:ee:c2","server_type":"test-type"}`
+	resp, _ = http.Post(ts.URL+"/api/hosts", "application/json", bytes.NewBufferString(body))
+	if resp.StatusCode != http.StatusConflict {
+		t.Fatalf("case-insensitive dup: got %d, want %d", resp.StatusCode, http.StatusConflict)
+	}
+	resp.Body.Close()
+}
+
 func itoa(i int64) string {
 	return fmt.Sprintf("%d", i)
 }