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/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(),
 		},
 		{