feat: Replace SSE with WebSocket for UI notifications (#36965)

* Closes #36942
* Fixes #19265 

Replaces the SSE-based push channel (`/user/events`) with a WebSocket
endpoint (`/-/ws`).

### What changes

- **New `/-/ws` endpoint** (authenticated). One WebSocket per origin,
shared across tabs via a single `SharedWorker`.
- **Pubsub broker** (`services/pubsub`) for fan-out by topic, behind a
`Broker` interface. `MemoryBroker` is the default (single process); a
Redis backend is available for multi-process setups, configured via
`[websocket].PUBSUB_TYPE` / `PUBSUB_CONN_STR`. The internal Gitea queue
was not usable here because it has FIFO/single-consumer semantics.
- **Push-only event production.** Events are emitted by write-triggered
notifiers — `NotificationCountChange`, `PublishStopwatchesForUser`, and
the logout publisher — wired into the existing `notify.Notifier`
interface. No server-side pollers.
- **Typed pub/sub on the client.** `web_src/js/modules/worker.ts` is a
singleton transport; features subscribe per event type via
`onUserEvent('notification-count', cb)` instead of branching on
`event.data.type`.
- **Wire contract** (`UserEventType` union) is shared between the worker
and consumers via `web_src/js/types.ts`, kept in sync with
`services/websocket/events.go`.
- **Client-side periodic polling fallback** kicks in only when the
WebSocket cannot be established (e.g. proxy blocks WS, browser lacks
module-SharedWorker support).

### What's removed

- `modules/eventsource` (SSE manager, run loop, messenger).
- `/user/events` route and `tests/integration/eventsource_test.go`.
- All server-side polling for stopwatches and notification counts.

### Stopwatch multi-tab fix

The navbar stopwatch icon was previously rendered conditionally on `{{if
$activeStopwatch}}`, so tabs loaded before the timer started had no DOM
element to update. The icon and popup are now always rendered (toggled
with `tw-hidden`), and the start/stop/cancel handlers POST silently so
all open tabs reflect the change in real time.

### Deployment note

WebSocket needs the upgrade headers to pass through a reverse proxy,
e.g. for nginx:

```nginx
proxy_set_header Upgrade $http_upgrade;
proxy_set_header Connection "upgrade";
```

Without them the WebSocket cannot be established, and after 3
consecutive failed opens the shared worker signals `push-unavailable`:
the notification count and stopwatch fall back to periodic polling on
the existing `[ui.notification]` timeouts. Real-time push is lost, the
features keep working. The reverse-proxy docs need the same note (see
the `docs-update-needed` label).

---------

Co-authored-by: silverwind <me@silverwind.io>
Co-authored-by: wxiaoguang <wxiaoguang@gmail.com>
Co-authored-by: Epid <rexmrj@gmail.com>

models/activities/notification_test.go

@@ -20,7 +20,8 @@ func TestCreateOrUpdateIssueNotifications(t *testing.T) {
 	assert.NoError(t, unittest.PrepareTestDatabase())
 	issue := unittest.AssertExistsAndLoadBean(t, &issues_model.Issue{ID: 1})
 
-	assert.NoError(t, activities_model.CreateOrUpdateIssueNotifications(t.Context(), issue.ID, 0, 2, 0))
+	_, err := activities_model.CreateOrUpdateIssueNotifications(t.Context(), issue.ID, 0, 2, 0)
+	assert.NoError(t, err)
 
 	// User 9 is inactive, thus notifications for user 1 and 4 are created
 	notf := unittest.AssertExistsAndLoadBean(t, &activities_model.Notification{UserID: 1, IssueID: issue.ID})
@@ -117,7 +118,8 @@ func TestUpdateNotificationStatuses(t *testing.T) {
 		&activities_model.Notification{UserID: user.ID, Status: activities_model.NotificationStatusRead})
 	notfPinned := unittest.AssertExistsAndLoadBean(t,
 		&activities_model.Notification{UserID: user.ID, Status: activities_model.NotificationStatusPinned})
-	assert.NoError(t, activities_model.UpdateNotificationStatuses(t.Context(), user, activities_model.NotificationStatusUnread, activities_model.NotificationStatusRead))
+	_, err := activities_model.UpdateNotificationStatuses(t.Context(), user, activities_model.NotificationStatusUnread, activities_model.NotificationStatusRead)
+	assert.NoError(t, err)
 	unittest.AssertExistsAndLoadBean(t,
 		&activities_model.Notification{ID: notfUnread.ID, Status: activities_model.NotificationStatusRead})
 	unittest.AssertExistsAndLoadBean(t,
@@ -131,7 +133,8 @@ func TestSetIssueReadBy(t *testing.T) {
 	user := unittest.AssertExistsAndLoadBean(t, &user_model.User{ID: 1})
 	issue := unittest.AssertExistsAndLoadBean(t, &issues_model.Issue{ID: 1})
 	assert.NoError(t, db.WithTx(t.Context(), func(ctx context.Context) error {
-		return activities_model.SetIssueReadBy(ctx, issue.ID, user.ID)
+		_, err := activities_model.SetIssueReadBy(ctx, issue.ID, user.ID)
+		return err
 	}))
 
 	nt, err := activities_model.GetIssueNotification(t.Context(), user.ID, issue.ID)