feat(demo): reset the demo instance every 20 minutes with a countdown

DEMO_RESET_MINUTES (default 20, 0 = off) resets the database to the
sample data. Demo accounts get fixed IDs so sessions survive; requests
hold a read lock so the reset never runs under a half-finished request
(streams excluded). DEMO_RESET goes to every connection, /auth/status
announces the next reset. The app shows "✨ Demo · Reset in MM:SS" in
place of the version, then clears cached answers, remounts the views,
reloads in the background and shows a short notice; if no event
arrives (long polling) it asks the server after the countdown.

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

internal/storage/users.go

@@ -67,6 +67,10 @@ func (s *Store) CreateUser(email, passwordHash, displayName, role string) (*doma
 }
 
 func (s *Store) CreateUserWithUsername(email, username, passwordHash, displayName, role string) (*domain.User, error) {
+	return s.createUserWithID(domain.NewID("usr"), email, username, passwordHash, displayName, role)
+}
+
+func (s *Store) createUserWithID(id, email, username, passwordHash, displayName, role string) (*domain.User, error) {
 	if role != "admin" && role != "member" {
 		role = "member"
 	}
@@ -76,7 +80,7 @@ func (s *Store) CreateUserWithUsername(email, username, passwordHash, displayNam
 		cleanUsername = strings.Split(cleanEmail, "@")[0]
 	}
 	u := &domain.User{
-		ID:                  domain.NewID("usr"),
+		ID:                  id,
 		Email:               cleanEmail,
 		Username:            cleanUsername,
 		PasswordHash:        passwordHash,
@@ -125,6 +129,13 @@ func (s *Store) GetUserByLogin(login string) (*domain.User, error) {
 }
 
 // EnsureDemoAccounts seeds admin/admin and user/user accounts for demo evaluation
+// Demo accounts keep the same IDs across resets, so the sessions of people trying the demo stay valid
+// when the database is reset underneath them.
+const (
+	DemoAdminID = "usr_demo_admin"
+	DemoUserID  = "usr_demo_user"
+)
+
 func (s *Store) EnsureDemoAccounts() error {
 	s.initMu.Lock()
 	defer s.initMu.Unlock()
@@ -142,7 +153,7 @@ func (s *Store) EnsureDemoAccounts() error {
 	adminCreated := false
 	adminUser, err := s.GetUserByLogin("admin")
 	if err != nil || adminUser == nil {
-		adminUser, err = s.CreateUserWithUsername("admin@mindsift.local", "admin", adminHash, "Admin", "admin")
+		adminUser, err = s.createUserWithID(DemoAdminID, "admin@mindsift.local", "admin", adminHash, "Admin", "admin")
 		if err != nil {
 			return fmt.Errorf("failed to create demo admin: %w", err)
 		}
@@ -155,7 +166,7 @@ func (s *Store) EnsureDemoAccounts() error {
 	// 2. Regular user (user / user)
 	normalUser, err := s.GetUserByLogin("user")
 	if err != nil || normalUser == nil {
-		normalUser, err = s.CreateUserWithUsername("user@mindsift.local", "user", userHash, "User", "member")
+		normalUser, err = s.createUserWithID(DemoUserID, "user@mindsift.local", "user", userHash, "User", "member")
 		if err != nil {
 			return fmt.Errorf("failed to create demo user: %w", err)
 		}