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/sentry_test.go

@@ -1,7 +1,6 @@
 package config_test
 
 import (
-	"os"
 	"testing"
 
 	"github.com/stretchr/testify/assert"
@@ -101,10 +100,10 @@ func TestEnvSentryDSN(t *testing.T) {
 		t.Run(tt.name, func(t *testing.T) {
 			// Cannot use t.Parallel() here because t.Setenv
 			// is incompatible with parallel subtests.
+			config.ClearEnvForTest(t)
+
 			if tt.set {
 				t.Setenv(envKeySentryDSN, tt.value)
-			} else {
-				require.NoError(t, os.Unsetenv(envKeySentryDSN))
 			}
 
 			got, err := config.EnvSentryDSNForTest(envKeySentryDSN)