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/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)