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

Config tests, and the first-boot debug log test that builds a Config,
start from an empty environment: config.ClearEnvForTest unsets every
variable the process has and restores each when the test ends, so
nothing exported in the developer's shell changes a result.
TestEnvPositiveInt and TestEnvPort share one table runner, and
TestEnvPort checks that the bad value appears in each error.

Every out-of-range PORT now wraps ErrInvalidPort: zero, negatives,
above 65535, and numbers too large or too small for an int.

The README configuration table and the Settings page say that a
RETENTION_SWEEP_INTERVAL that does not parse, or is zero or negative,
fails startup.

Model: opus-5-5

internal/config/testing.go

@@ -0,0 +1,30 @@
+package config
+
+import (
+	"os"
+	"strings"
+	"testing"
+)
+
+// ClearEnvForTest unsets every variable in the process environment
+// for the rest of the test and puts each back when the test ends, so
+// a test sees only the variables it sets itself, not whatever the
+// developer's shell exports.
+func ClearEnvForTest(t *testing.T) {
+	t.Helper()
+
+	for _, entry := range os.Environ() {
+		key, _, _ := strings.Cut(entry, "=")
+
+		// 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)
+		}
+	}
+}