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>

modules/json/json.go

@@ -23,6 +23,7 @@ type Decoder interface {
 // Interface represents an interface to handle json data
 type Interface interface {
 	Marshal(v any) ([]byte, error)
+	MarshalWrite(w io.Writer, v any) error
 	Unmarshal(data []byte, v any) error
 	NewEncoder(writer io.Writer) Encoder
 	NewDecoder(reader io.Reader) Decoder
@@ -36,6 +37,11 @@ func Marshal(v any) ([]byte, error) {
 	return DefaultJSONHandler.Marshal(v)
 }
 
+// MarshalWrite writes the JSON encoding of v to the given writer
+func MarshalWrite(w io.Writer, v any) error {
+	return DefaultJSONHandler.MarshalWrite(w, v)
+}
+
 // Unmarshal decodes object from bytes
 func Unmarshal(data []byte, v any) error {
 	return DefaultJSONHandler.Unmarshal(data, v)

modules/json/jsonv1.go

@@ -17,6 +17,10 @@ func (jsonV1) Marshal(v any) ([]byte, error) {
 	return json.Marshal(v)
 }
 
+func (jsonV1) MarshalWrite(w io.Writer, v any) error {
+	return json.NewEncoder(w).Encode(v)
+}
+
 func (jsonV1) Unmarshal(data []byte, v any) error {
 	return json.Unmarshal(data, v)
 }

modules/json/jsonv2.go

@@ -53,6 +53,10 @@ func (j *JSONv2) Marshal(v any) ([]byte, error) {
 	return jsonv2.Marshal(v, j.marshalOptions)
 }
 
+func (j *JSONv2) MarshalWrite(w io.Writer, v any) error {
+	return jsonv2.MarshalWrite(w, v, j.marshalOptions)
+}
+
 func (j *JSONv2) Unmarshal(data []byte, v any) error {
 	return jsonv2.Unmarshal(data, v, j.unmarshalOptions)
 }