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/config/config.go

@@ -80,8 +80,7 @@ const (
 	// process over a Docker network or a private LAN connects from.
 	defaultTrustedProxies = "10.0.0.0/8,172.16.0.0/12,192.168.0.0/16"
 
-	// maxPort is the highest valid TCP port number. The lower
-	// bound (at least 1) is enforced by envPositiveInt.
+	// maxPort is the highest valid TCP port number.
 	maxPort = 65535
 
 	// mappedV4Offset is the number of leading bits an IPv4-mapped
@@ -105,7 +104,7 @@ var ErrInvalidEnvironment = errors.New("invalid environment")
 var ErrNonPositiveValue = errors.New("value must be positive")
 
 // ErrInvalidPort is returned when an environment variable holding a
-// TCP port number is set above the valid port range.
+// TCP port number is set to a number outside 1 to 65535.
 var ErrInvalidPort = errors.New("invalid port")
 
 // ErrInvalidCIDR is returned when an environment variable holding a
@@ -363,17 +362,25 @@ func envPositiveInt(
 // envPort returns the value of the named environment variable parsed
 // as a TCP port number. Returns defaultValue if not set. A set value
 // that is unparseable, below 1, or above maxPort is a hard error
-// naming the key and the bad value.
+// naming the key and the bad value; the two out-of-range cases both
+// wrap ErrInvalidPort.
 func envPort(key string, defaultValue int) (int, error) {
-	port, err := envPositiveInt(key, defaultValue)
-	if err != nil {
-		return 0, err
+	v := os.Getenv(key)
+	if v == "" {
+		return defaultValue, nil
 	}
 
-	if port > maxPort {
+	port, err := strconv.Atoi(v)
+	if err != nil {
 		return 0, fmt.Errorf(
-			"%w: %s must be at most %d, got %d",
-			ErrInvalidPort, key, maxPort, port,
+			"invalid integer for %s: %q: %w", key, v, err,
+		)
+	}
+
+	if port < 1 || port > maxPort {
+		return 0, fmt.Errorf(
+			"%w: %s must be from 1 to %d, got %q",
+			ErrInvalidPort, key, maxPort, v,
 		)
 	}