Remove the no-op MAINTENANCE_MODE setting (closes #317)

MAINTENANCE_MODE did nothing but make the healthcheck JSON report maintenanceMode: true; no request was ever served differently, so an operator who set it expecting requests to be refused got nothing. It is removed from the configuration, the startup configuration log line, the healthcheck JSON, the README and the Settings page, together with the uncalled Server.MaintenanceMode method and the healthcheck's dependency on the configuration. A leftover value in an environment is ignored like any other unknown variable.

Model: opus-5-5

internal/config/env_test.go

@@ -18,10 +18,9 @@ const testEnvKey = "WEBHOOKER_TEST_VALUE"
 
 // Real configuration variables exercised by the config.New tests.
 const (
-	envKeyPort            = "PORT"
-	envKeyDebug           = "DEBUG"
-	envKeyMaintenanceMode = "MAINTENANCE_MODE"
-	envKeyBindAddress     = "BIND_ADDRESS"
+	envKeyPort        = "PORT"
+	envKeyDebug       = "DEBUG"
+	envKeyBindAddress = "BIND_ADDRESS"
 )
 
 // Sample BIND_ADDRESS values used by the tables below.
@@ -604,12 +603,6 @@ func flagEnvValueCases() []badEnvValueCase {
 			value:       "ture",
 			expectError: true,
 		},
-		{
-			name:        "unparseable MAINTENANCE_MODE aborts startup",
-			key:         envKeyMaintenanceMode,
-			value:       "sometimes",
-			expectError: true,
-		},
 	}
 }
 
@@ -656,8 +649,7 @@ func TestNewUsesDefaultsWhenUnset(t *testing.T) {
 	t.Setenv("WEBHOOKER_ENVIRONMENT", "dev")
 
 	for _, key := range []string{
-		envKeyPort, envKeyDebug, envKeyMaintenanceMode,
-		envKeyBindAddress, envKeySentryDSN,
+		envKeyPort, envKeyDebug, envKeyBindAddress, envKeySentryDSN,
 	} {
 		require.NoError(t, os.Unsetenv(key))
 	}
@@ -668,7 +660,6 @@ func TestNewUsesDefaultsWhenUnset(t *testing.T) {
 
 	assert.Equal(t, 8080, cfg.Port)
 	assert.False(t, cfg.Debug)
-	assert.False(t, cfg.MaintenanceMode)
 
 	// Loopback, not the wildcard: the default must not publish the
 	// cleartext admin UI and the unauthenticated receiver on every