fix(server): stop the background compaction before the database is closed

The compaction starts right away in NewServer and could still write to the SQLite files while a
test's temp dir was removed ("directory not empty" on the CI runner, TestWebhookTokenInURLOnlyAPIKey).
Server.Close stops it and waits for a running pass; tests call it before closing the store, and the
server calls it on shutdown before the deferred store.Close. v0.37.1.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

internal/api/server.go

@@ -7,6 +7,7 @@ import (
 	"log/slog"
 	"net/http"
 	"strings"
+	"sync"
 	"time"
 
 	"github.com/go-chi/chi/v5"
@@ -41,6 +42,11 @@ type Server struct {
 	notifyPushHook    func(userID string, msg pushMessage) error
 	pushHTTPClient    *http.Client // tests: reach a local push service
 	push              pushKeys
+
+	// Background work that touches the database right from the start (compaction); Close stops it
+	stop     chan struct{}
+	stopOnce sync.Once
+	bg       sync.WaitGroup
 }
 
 func NewServer(cfg *config.Config, store *storage.Store, wsHub *ws.Hub, staticFS fs.FS) *Server {
@@ -51,6 +57,7 @@ func NewServer(cfg *config.Config, store *storage.Store, wsHub *ws.Hub, staticFS
 		staticFS: staticFS,
 		leases:   collab.NewLeaseManager(nil),
 		notifier: collab.NewNotifier(),
+		stop:     make(chan struct{}),
 	}
 	srv.loginGuard = newLoginGuard(!cfg.DisableLoginProtection)
 	srv.voice = newVoiceService(cfg.Speech.MaxParallel)
@@ -63,6 +70,7 @@ func NewServer(cfg *config.Config, store *storage.Store, wsHub *ws.Hub, staticFS
 	store.OnTaskCreated = func(th *domain.Thought) {
 		srv.wsHub.BroadcastToUsers(srv.taskAudience(th), "TASK_CREATED", th)
 	}
+	srv.bg.Add(1)
 	go srv.startCompaction()
 	if cfg.DemoMode && cfg.DemoResetMinutes > 0 {
 		go srv.startDemoResetTicker()
@@ -93,6 +101,13 @@ func (s *Server) taskAudience(th *domain.Thought) []string {
 	return append(s.store.GetTaskListUserIDs(th.TaskListID), th.UserID)
 }
 
+// Close stops the background compaction and waits until a running pass has finished, so the database
+// can be closed safely afterwards (tests close it right away; a pass still writing would race with that).
+func (s *Server) Close() {
+	s.stopOnce.Do(func() { close(s.stop) })
+	s.bg.Wait()
+}
+
 func (s *Server) Router() http.Handler {
 	r := chi.NewRouter()