feat(delivery): support multi-instance scheduling on a single host

- Per-host instance limit (DELIVERY_HOST_INSTANCE_LIMIT, default 4)
- Optional target_host to pin a host from the candidate pool
- Regenerate swagger docs

server/internal/service/delivery_test.go

@@ -15,6 +15,7 @@ func TestValidateDeliveryInput(t *testing.T) {
 	full.Topology = "standalone"
 	full.MySQLPort = 13306
 	full.DataDisk = "/disk1"
+	full.TargetHost = "k8s-server-03"
 	full.MemoryMi = 8192
 	full.StorageGi = 2000
 	full.Timezone = "+08:00"
@@ -55,6 +56,7 @@ func TestValidateDeliveryInput(t *testing.T) {
 		"port below pool":       func(in *MySQLDeliveryInput) { in.MySQLPort = 3307 },
 		"port above pool":       func(in *MySQLDeliveryInput) { in.MySQLPort = 14000 },
 		"data disk not in list": func(in *MySQLDeliveryInput) { in.DataDisk = "/mnt/other" },
+		"bad target host":       func(in *MySQLDeliveryInput) { in.TargetHost = "-bad-host" },
 		"bad timezone":          func(in *MySQLDeliveryInput) { in.Timezone = "UTC+8" },
 		"bad lower case":        func(in *MySQLDeliveryInput) { in.LowerCaseTableNames = intPtr(2) },
 		"bad charset":           func(in *MySQLDeliveryInput) { in.CharacterSet = "big5" },
@@ -77,6 +79,30 @@ func TestValidateDeliveryInput(t *testing.T) {
 	}
 }
 
+func TestFirstFreeHost(t *testing.T) {
+	hosts := []targetHost{{Name: "node-a"}, {Name: "node-b"}}
+	if h := firstFreeHost(hosts, nil, 1); h == nil || h.Name != "node-a" {
+		t.Fatalf("expected node-a on empty occupancy, got %+v", h)
+	}
+	if h := firstFreeHost(hosts, []string{"node-a"}, 1); h == nil || h.Name != "node-b" {
+		t.Fatalf("expected node-b when node-a is full at limit 1, got %+v", h)
+	}
+	if h := firstFreeHost(hosts, []string{"node-a", "node-b"}, 1); h != nil {
+		t.Fatalf("limit 1 with all hosts taken should return nil, got %+v", h)
+	}
+	// 摊满一轮后回到首台叠加第二个实例
+	if h := firstFreeHost(hosts, []string{"node-a", "node-b"}, 2); h == nil || h.Name != "node-a" {
+		t.Fatalf("expected node-a for second round at limit 2, got %+v", h)
+	}
+	if h := firstFreeHost(hosts, []string{"node-a", "node-a", "node-b", "node-b"}, 2); h != nil {
+		t.Fatalf("limit 2 with all hosts saturated should return nil, got %+v", h)
+	}
+	// limit < 1 退化为一机一实例
+	if h := firstFreeHost(hosts, []string{"node-a"}, 0); h == nil || h.Name != "node-b" {
+		t.Fatalf("limit 0 should degrade to 1, got %+v", h)
+	}
+}
+
 func TestAllocatePort(t *testing.T) {
 	if port, err := allocatePort(0, nil); err != nil || port != mysqlPortPoolStart {
 		t.Fatalf("expected first pool port %d, got %d err=%v", mysqlPortPoolStart, port, err)