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

Config tests that unset a variable now restore it when they end, and
every test that builds a Config first unsets every variable the config
reads, so a value exported in the developer's shell cannot change the
result. TestEnvPort checks that the bad value appears in its errors.

A PORT of zero or below now wraps ErrInvalidPort, as one above 65535
already did.

The README configuration table and the Settings page now say that a
RETENTION_SWEEP_INTERVAL that does not parse, or is zero or negative,
fails 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"
@@ -104,7 +103,7 @@ func TestEnvSentryDSN(t *testing.T) {
 			if tt.set {
 				t.Setenv(envKeySentryDSN, tt.value)
 			} else {
-				require.NoError(t, os.Unsetenv(envKeySentryDSN))
+				unsetEnv(t, envKeySentryDSN)
 			}
 
 			got, err := config.EnvSentryDSNForTest(envKeySentryDSN)