Fail loudly on set-but-unparseable env config values (closes #80) (#92)

Defaults now apply only to unset or empty environment variables; a set-but-
unparseable value aborts startup with an error naming the key and the value.
envInt is gone, envBool parses with strconv.ParseBool, and PORT is bounded.

internal/config/export_test.go

@@ -0,0 +1,20 @@
+package config
+
+// This file exposes the unexported environment parsing helpers to
+// the external config_test package so each helper can be covered by
+// its own table-driven test without weakening the package API.
+
+// EnvBoolForTest exposes envBool.
+func EnvBoolForTest(key string, defaultValue bool) (bool, error) {
+	return envBool(key, defaultValue)
+}
+
+// EnvPositiveIntForTest exposes envPositiveInt.
+func EnvPositiveIntForTest(key string, defaultValue int) (int, error) {
+	return envPositiveInt(key, defaultValue)
+}
+
+// EnvPortForTest exposes envPort.
+func EnvPortForTest(key string, defaultValue int) (int, error) {
+	return envPort(key, defaultValue)
+}