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/gormlog/firstboot_test.go

@@ -117,8 +117,8 @@ func readFirstBootSecrets(
 }
 
 // bootAtDebug starts and stops the real application graph against
-// dataDir with DEBUG=true, and returns everything it wrote to standard
-// output.
+// dataDir with DEBUG=true and nothing else set, and returns everything
+// it wrote to standard output.
 //
 // config.New reads DEBUG from the environment exactly as the binary
 // does, internal/logger builds the handler it builds in production,
@@ -128,6 +128,7 @@ func readFirstBootSecrets(
 func bootAtDebug(t *testing.T, dataDir string) string {
 	t.Helper()
 
+	config.ClearEnvForTest(t)
 	t.Setenv("DEBUG", "true")
 	t.Setenv("DATA_DIR", dataDir)