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/healthcheck/healthcheck.go

@@ -7,7 +7,6 @@ import (
 	"time"
 
 	"go.uber.org/fx"
-	"sneak.berlin/go/webhooker/internal/config"
 	"sneak.berlin/go/webhooker/internal/database"
 	"sneak.berlin/go/webhooker/internal/globals"
 	"sneak.berlin/go/webhooker/internal/logger"
@@ -18,7 +17,6 @@ type HealthcheckParams struct {
 	fx.In
 
 	Globals  *globals.Globals
-	Config   *config.Config
 	Logger   *logger.Logger
 	Database *database.Database
 }
@@ -64,7 +62,6 @@ func (s *Healthcheck) Healthcheck() *Response {
 		UptimeHuman:   s.uptime().String(),
 		Appname:       s.params.Globals.Appname,
 		Version:       s.params.Globals.Version,
-		Maintenance:   s.params.Config.MaintenanceMode,
 	}
 
 	return resp
@@ -78,7 +75,6 @@ type Response struct {
 	UptimeHuman   string `json:"uptimeHuman"`
 	Version       string `json:"version"`
 	Appname       string `json:"appname"`
-	Maintenance   bool   `json:"maintenanceMode"`
 }
 
 func (s *Healthcheck) uptime() time.Duration {