Isolate config tests from the shell; any out-of-range PORT is ErrInvalidPort (closes #94)

The config tests unset variables without restoring them and read whatever the developer's shell exported, so results could differ from one machine to the next. config.ClearEnvForTest, in internal/config/testing.go, unsets every variable in the process environment and, when the test ends, leaves it exactly as it found it; every config test and the first-boot test call it before setting their own. TestEnvPositiveInt and TestEnvPort share one table runner, and the port errors name the bad value. A PORT of zero, below zero, or too large to parse now matches ErrInvalidPort, as one above 65535 already did. The README and the Settings page say an unparseable or non-positive RETENTION_SWEEP_INTERVAL stops startup.

Model: opus-5-5

internal/config/testing.go

@@ -0,0 +1,50 @@
+package config
+
+import (
+	"os"
+	"strings"
+	"testing"
+)
+
+// ClearEnvForTest unsets every variable in the process environment
+// for the rest of the test, so a test sees only the variables it sets
+// itself, not whatever the developer's shell exports. When the test
+// ends it leaves the environment exactly as it found it: each variable
+// it unset is put back, and any variable added since is removed.
+func ClearEnvForTest(t *testing.T) {
+	t.Helper()
+
+	present := make(map[string]bool)
+
+	for _, entry := range os.Environ() {
+		key, _, _ := strings.Cut(entry, "=")
+		present[key] = true
+
+		// t.Setenv registers the restore; the Unsetenv after it is
+		// what makes the key absent, since a key set to the empty
+		// string is still present, and godotenv will not overwrite a
+		// present key.
+		t.Setenv(key, "")
+
+		err := os.Unsetenv(key)
+		if err != nil {
+			t.Fatalf("unsetting %s: %v", key, err)
+		}
+	}
+
+	// A variable the test adds other than through t.Setenv, as loading
+	// a .env file does, has no restore of its own.
+	t.Cleanup(func() {
+		for _, entry := range os.Environ() {
+			key, _, _ := strings.Cut(entry, "=")
+			if present[key] {
+				continue
+			}
+
+			err := os.Unsetenv(key)
+			if err != nil {
+				t.Errorf("unsetting %s: %v", key, err)
+			}
+		}
+	})
+}