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

MAINTENANCE_MODE only made the healthcheck report
maintenanceMode: true; no request was ever served differently. It is
removed from the configuration, the healthcheck JSON, the README and
the Settings page, together with the unused Server.MaintenanceMode
method and the healthcheck's Config dependency, which existed only to
read it. A leftover MAINTENANCE_MODE in the environment is not read,
so it is ignored rather than refused.

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