Refuse invalid settings at start, health check follows PORT (closes #31)

PORT (1 to 65535) and XDG_DATA_HOME (an absolute path) are now checked in
the config package, which is built before the database opens, so a bad
value stops the start with an error and a non-zero exit instead of leaving
the daemon running without HTTP or putting the database somewhere else.
entrypoint.sh refuses a MALLOC_ARENA_MAX that is not a positive whole
number, since glibc ignores a bad one silently. The HEALTHCHECK probes the
port PORT names. The README corrects the Linux state directory, lists
XDG_DATA_HOME, and adds a "Running under upaas" section.

Model: opus-5-5

internal/config/config_test.go

@@ -0,0 +1,62 @@
+package config
+
+import (
+	"runtime"
+	"testing"
+)
+
+func TestNewReadsPort(t *testing.T) {
+	tests := map[string]int{
+		"":      defaultPort,
+		"1":     1,
+		"9090":  9090,
+		"65535": 65535,
+	}
+
+	for value, want := range tests {
+		t.Run(value, func(t *testing.T) {
+			t.Setenv("PORT", value)
+			t.Setenv("XDG_DATA_HOME", "")
+
+			cfg, err := New()
+			if err != nil {
+				t.Fatalf("New() with PORT=%q: %v", value, err)
+			}
+
+			if cfg.Port != want {
+				t.Errorf("New() with PORT=%q: Port = %d, want %d", value, cfg.Port, want)
+			}
+		})
+	}
+}
+
+func TestNewRefusesInvalidPort(t *testing.T) {
+	for _, value := range []string{"0", "65536", "-1", "http", "80.5"} {
+		t.Run(value, func(t *testing.T) {
+			t.Setenv("PORT", value)
+			t.Setenv("XDG_DATA_HOME", "")
+
+			if _, err := New(); err == nil {
+				t.Errorf("New() with PORT=%q returned no error", value)
+			}
+		})
+	}
+}
+
+func TestNewRefusesRelativeXDGDataHome(t *testing.T) {
+	if runtime.GOOS == "darwin" {
+		t.Skip("macOS does not read XDG_DATA_HOME")
+	}
+
+	t.Setenv("PORT", "")
+
+	t.Setenv("XDG_DATA_HOME", "relative/path")
+	if _, err := New(); err == nil {
+		t.Error("New() with a relative XDG_DATA_HOME returned no error")
+	}
+
+	t.Setenv("XDG_DATA_HOME", "/var/lib")
+	if _, err := New(); err != nil {
+		t.Errorf("New() with XDG_DATA_HOME=/var/lib: %v", err)
+	}
+}