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.go

@@ -336,30 +336,33 @@ func GetUIDsAndNotificationCounts(ctx context.Context, since, until timeutil.Tim
 	return res, db.GetEngine(ctx).SQL(sql, NotificationStatusUnread, since, until).Find(&res)
 }
 
-// SetIssueReadBy sets issue to be read by given user.
-func SetIssueReadBy(ctx context.Context, issueID, userID int64) error {
+// SetIssueReadBy sets issue to be read by given user. The bool result is true
+// when the unread count actually decreased, so callers can skip a push on no-op.
+func SetIssueReadBy(ctx context.Context, issueID, userID int64) (bool, error) {
 	if err := issues_model.UpdateIssueUserByRead(ctx, userID, issueID); err != nil {
-		return err
+		return false, err
 	}
 
 	return setIssueNotificationStatusReadIfUnread(ctx, userID, issueID)
 }
 
-func setIssueNotificationStatusReadIfUnread(ctx context.Context, userID, issueID int64) error {
+func setIssueNotificationStatusReadIfUnread(ctx context.Context, userID, issueID int64) (bool, error) {
 	notification, err := GetIssueNotification(ctx, userID, issueID)
 	// ignore if not exists
 	if err != nil {
-		return nil
+		return false, nil
 	}
 
 	if notification.Status != NotificationStatusUnread {
-		return nil
+		return false, nil
 	}
 
 	notification.Status = NotificationStatusRead
 
-	_, err = db.GetEngine(ctx).ID(notification.ID).Cols("status").Update(notification)
-	return err
+	if _, err := db.GetEngine(ctx).ID(notification.ID).Cols("status").Update(notification); err != nil {
+		return false, err
+	}
+	return true, nil
 }
 
 // SetRepoReadBy sets repo to be visited by given user.
@@ -407,12 +410,13 @@ func GetNotificationByID(ctx context.Context, notificationID int64) (*Notificati
 	return notification, nil
 }
 
-// UpdateNotificationStatuses updates the statuses of all of a user's notifications that are of the currentStatus type to the desiredStatus
-func UpdateNotificationStatuses(ctx context.Context, user *user_model.User, currentStatus, desiredStatus NotificationStatus) error {
+// UpdateNotificationStatuses updates the statuses of all of a user's notifications
+// that are of the currentStatus type to the desiredStatus. Returns the number of
+// rows actually changed so callers can skip downstream work on a no-op.
+func UpdateNotificationStatuses(ctx context.Context, user *user_model.User, currentStatus, desiredStatus NotificationStatus) (int64, error) {
 	n := &Notification{Status: desiredStatus, UpdatedBy: user.ID}
-	_, err := db.GetEngine(ctx).
+	return db.GetEngine(ctx).
 		Where("user_id = ? AND status = ?", user.ID, currentStatus).
 		Cols("status", "updated_by", "updated_unix").
 		Update(n)
-	return err
 }