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/handlers/settings.go

@@ -77,7 +77,8 @@ func settingRows(cfg *config.Config) []settingRow {
 		{
 			"RETENTION_SWEEP_INTERVAL",
 			"How often the retention reaper and archive sweeper run " +
-				"(Go duration, must be positive)",
+				"(Go duration, must be positive). A value that does " +
+				"not parse, or is zero or negative, fails startup",
 			cfg.RetentionSweepInterval.String(),
 		},
 		{