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/dotenv_test.go

@@ -22,17 +22,6 @@ const malformedDotEnv = "PORT 19615\n" +
 	"this is not = valid ! syntax\n" +
 	"\"unclosed\n"
 
-// unsetDotEnvKey makes dotEnvKey genuinely absent for the duration of
-// the test and restores it afterwards. t.Setenv registers the restore;
-// the Unsetenv that follows is what the test actually needs, because a
-// variable set to the empty string is still present in os.Environ and
-// godotenv would refuse to overwrite it.
-func unsetDotEnvKey(t *testing.T) {
-	t.Helper()
-	t.Setenv(dotEnvKey, "placeholder")
-	require.NoError(t, os.Unsetenv(dotEnvKey))
-}
-
 // writeDotEnv writes contents to a .env file in a fresh temporary
 // directory and returns its path.
 func writeDotEnv(t *testing.T, contents string) string {
@@ -50,9 +39,9 @@ func writeDotEnv(t *testing.T, contents string) string {
 // normally rather than be refused for a file it was never meant to
 // have.
 //
-//nolint:paralleltest // unsetDotEnvKey uses t.Setenv.
+//nolint:paralleltest // ClearEnvForTest uses t.Setenv.
 func TestLoadDotEnv_MissingFileIsFine(t *testing.T) {
-	unsetDotEnvKey(t)
+	config.ClearEnvForTest(t)
 
 	absent := filepath.Join(t.TempDir(), config.DotEnvPath)
 	require.NoError(t, config.LoadDotEnvFileForTest(absent))
@@ -65,9 +54,9 @@ func TestLoadDotEnv_MissingFileIsFine(t *testing.T) {
 // reaches the environment, which is the whole reason the file is read
 // at all.
 //
-//nolint:paralleltest // unsetDotEnvKey uses t.Setenv.
+//nolint:paralleltest // ClearEnvForTest uses t.Setenv.
 func TestLoadDotEnv_AppliesValues(t *testing.T) {
-	unsetDotEnvKey(t)
+	config.ClearEnvForTest(t)
 
 	path := writeDotEnv(t, "# a comment\n"+dotEnvKey+"=from-dot-env\n")
 
@@ -93,9 +82,9 @@ func TestLoadDotEnv_RealEnvironmentWins(t *testing.T) {
 // reverts to its default; the process used to start that way with no
 // log line naming the file at all.
 //
-//nolint:paralleltest // unsetDotEnvKey uses t.Setenv.
+//nolint:paralleltest // ClearEnvForTest uses t.Setenv.
 func TestLoadDotEnv_MalformedFileAborts(t *testing.T) {
-	unsetDotEnvKey(t)
+	config.ClearEnvForTest(t)
 
 	path := writeDotEnv(
 		t, malformedDotEnv+dotEnvKey+"=from-dot-env\n",
@@ -143,7 +132,7 @@ func TestLoadDotEnv_UnreadableFileAborts(t *testing.T) {
 //
 //nolint:paralleltest // t.Chdir moves the whole process.
 func TestLoadDotEnv_ReadsTheWorkingDirectory(t *testing.T) {
-	unsetDotEnvKey(t)
+	config.ClearEnvForTest(t)
 
 	dir := t.TempDir()
 	require.NoError(t, os.WriteFile(