Add Discord guild ID configuration and enhance event announcement options

This commit introduces the `DISCORD_GUILD_ID` environment variable to the configuration files, allowing for better integration with Discord for role synchronization. The event announcement functionality has been updated to include an option for the `@everyone` mention, which can be toggled during event creation. The backend logic has been modified to handle this new option, and corresponding updates have been made to the frontend to allow users to control the mention behavior. Additionally, tests have been added to ensure the correct functionality of these features.

frontend/src/App.tsx

@@ -353,11 +353,15 @@ function EventSetup({ event, onCreated, onDeleted }: { event?: MixEvent; onCreat
   const language = useLanguage()
   const initialForm = useMemo(() => eventInput(event), [event])
   const [form, setForm] = useState<EventInput>(initialForm)
-  useEffect(() => setForm(initialForm), [initialForm])
+  const [pingEveryone, setPingEveryone] = useState(true)
+  useEffect(() => {
+    setForm(initialForm)
+    setPingEveryone(true)
+  }, [initialForm])
   const save = useMutation({
     mutationFn: () => {
       const payload = { ...form, startsAt: new Date(form.startsAt).toISOString(), endsAt: new Date(form.endsAt).toISOString(), registrationDeadline: new Date(form.registrationDeadline).toISOString() }
-      return event ? api.updateEvent(event.id, payload) : api.createEvent(payload)
+      return event ? api.updateEvent(event.id, payload) : api.createEvent({ ...payload, pingEveryone })
     },
     onSuccess: (saved) => {
       void client.invalidateQueries({ queryKey: ['events'] })
@@ -404,6 +408,7 @@ function EventSetup({ event, onCreated, onDeleted }: { event?: MixEvent; onCreat
       <div className="section-title"><div><h2>{event ? 'Event settings' : 'New event'}</h2><p>Scheduling and registration</p></div>{event && <div className="event-admin-actions"><Badge tone={event.status === 'cancelled' ? 'danger' : event.status === 'live' ? 'live' : 'success'}>{event.status === 'cancelled' ? 'Cancelled' : event.status === 'completed' ? 'Completed' : event.status === 'live' ? 'LIVE' : 'Published'}</Badge><button className="button danger-outline" type="button" disabled={cancel.isPending || event.status === 'cancelled' || event.status === 'completed'} onClick={() => cancel.mutate()}>Cancel event</button><button className="icon-button delete-event" type="button" disabled={remove.isPending} aria-label="Delete event" title="Delete event" onClick={() => { if (window.confirm(language === 'ru' ? 'Удалить это событие навсегда?' : 'Delete this event permanently?')) remove.mutate() }}><X /></button></div>}</div>
       <div className="field-grid"><label>Event name<input required value={form.name} onChange={(e) => set('name', e.target.value)} /></label><label>Start time<input required type="datetime-local" value={form.startsAt} onChange={(e) => setStart(e.target.value)} /></label><label>End time<input required type="datetime-local" value={form.endsAt} onChange={(e) => set('endsAt', e.target.value)} /></label><label>Registration deadline<input required readOnly type="datetime-local" value={form.registrationDeadline} /></label></div>
       <label>Description<textarea value={form.description} onChange={(e) => set('description', e.target.value)} /></label>
+      {!event && <label className="announcement-toggle"><input type="checkbox" checked={pingEveryone} onChange={(e) => setPingEveryone(e.target.checked)} /><span><strong>Ping everyone in Discord</strong><small>The announcement is always posted; this controls the @everyone notification.</small></span></label>}
       <div className="form-footer"><p>{save.isError ? save.error.message : cancel.isError ? cancel.error.message : remove.isError ? remove.error.message : 'Times are sent to the server in UTC.'}</p><button className="button primary" disabled={save.isPending}>{save.isPending ? 'Saving…' : event ? 'Save changes' : 'Create event'}</button></div>
     </form>
     <aside className="card admin-summary"><h2>Server authority</h2><div className="readiness-score"><Shield /><span>Validation runs on the backend</span></div><ul><li><Check />UTC scheduling</li><li><Check />Audited admin changes</li><li><Check />Registration deadline rules</li></ul></aside>