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, when the test ends, leaves the
environment exactly as it found it, so nothing exported in the
developer's shell changes a result and nothing a test sets outlives it.
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/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)