fix(project_events): use process-lifetime ctx for async publish

The publish goroutine inherited the request context via
context.WithoutCancel. That context carries a request-scoped DB
session returned to the pool when the HTTP handler completes, so
GetProjectByID + access checks in the goroutine raced with session
teardown and intermittently returned empty recipient sets (uids=[]),
silently dropping every SSE board event. Root the detached context in
graceful.ShutdownContext() (global engine, process lifetime).

(cherry picked from commit bfc10289e6)

services/project_events/events.go

@@ -22,6 +22,7 @@ import (
 	"code.gitea.io/gitea/models/unit"
 	user_model "code.gitea.io/gitea/models/user"
 	"code.gitea.io/gitea/modules/eventsource"
+	"code.gitea.io/gitea/modules/graceful"
 	"code.gitea.io/gitea/modules/json"
 	"code.gitea.io/gitea/modules/log"
 )
@@ -317,8 +318,16 @@ func PublishProjectDeleted(ctx context.Context, payload ProjectDeleted) {
 	go publishEvent(detach(ctx), payload.ProjectID, payload)
 }
 
-// detach strips cancellation/deadline from ctx but preserves stored
-// values (notably the session tag) so the goroutine outlives the request.
-func detach(ctx context.Context) context.Context {
-	return context.WithoutCancel(ctx)
+// detach returns a context safe for use in the fire-and-forget publish
+// goroutine. The request's context carries a request-scoped DB session
+// that is returned to the pool once the HTTP handler completes; reusing
+// it from the goroutine races with that teardown and makes subsequent
+// queries (GetProjectByID, access checks) fail intermittently. The
+// session tag is already resolved synchronously before the goroutine
+// starts, so the goroutine needs no request-scoped values — only a
+// clean, process-lifetime DB context. ShutdownContext is backed by the
+// global engine, outlives any single request, and is cancelled on app
+// shutdown so we don't leak goroutines past teardown.
+func detach(_ context.Context) context.Context {
+	return graceful.GetManager().ShutdownContext()
 }