DEBUG turns debug logging on for any non-empty value (closes #159)

DEBUG was read with strconv.ParseBool, so a value such as "yes" stopped
the start. It now follows the code style guide: unset or empty is off,
and any other value, "false" and "0" included, turns debug logging on.
No value of DEBUG stops the start. The test that "yes" fails is replaced
by one covering unset, empty, "false", "0" and "yes". README.md no
longer gives false as the default, and says which values turn it on.

Model: opus-5-5

internal/config/config.go

@@ -41,7 +41,6 @@ var (
 		"must be an origin, scheme://host with an optional port",
 	)
 	errNotPort = errors.New("must be a port number, 1 to 65535")
-	errNotBool = errors.New("must be true or false")
 	errNotIP   = errors.New("must be an IP address, or empty")
 
 	errMetricsCredentials = errors.New(
@@ -101,7 +100,7 @@ func New(
 	viper.SetDefault("CORS_ALLOWED_ORIGINS", "")
 	viper.SetDefault("DATA_DIR", "./data/reports")
 	viper.SetDefault("DATA_DIR_MAX_BYTES", defaultDataDirMaxBytes)
-	viper.SetDefault("DEBUG", "false")
+	viper.SetDefault("DEBUG", "")
 	// An empty BIND_ADDRESS listens on every interface.
 	viper.SetDefault("BIND_ADDRESS", "")
 	viper.SetDefault("PORT", "8080")
@@ -120,19 +119,17 @@ func New(
 		}
 	}
 
-	// Read with strconv: viper's GetInt and GetBool would read a value
-	// they cannot parse as 0 or false instead of failing.
+	// Read with strconv: viper's GetInt would read a value it cannot
+	// parse as 0 instead of failing.
 	port, err := strconv.Atoi(viper.GetString("PORT"))
 	if err != nil || port < 1 || port > math.MaxUint16 {
 		return nil, fmt.Errorf("PORT %q: %w",
 			viper.GetString("PORT"), errNotPort)
 	}
 
-	debug, err := strconv.ParseBool(viper.GetString("DEBUG"))
-	if err != nil {
-		return nil, fmt.Errorf("DEBUG %q: %w",
-			viper.GetString("DEBUG"), errNotBool)
-	}
+	// Any non-empty value turns debug logging on, "false" and "0"
+	// included, as the code style guide says.
+	debug := viper.GetString("DEBUG") != ""
 
 	// An empty TRUSTED_PROXIES trusts no proxy, but viper reads an
 	// empty environment variable as unset and would give the default,
@@ -172,8 +169,8 @@ func New(
 }
 
 // check fails with an error naming the first setting here whose value
-// the server cannot use. New checks PORT and DEBUG as it reads them,
-// and the middleware checks TRUSTED_PROXIES as it parses it.
+// the server cannot use. New checks PORT as it reads it, and the
+// middleware checks TRUSTED_PROXIES as it parses it.
 func (s *Config) check() error {
 	// viper reads a value that is not a number as 0, so this also
 	// catches a mistyped setting.