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/api/demo.go

@@ -0,0 +1,77 @@
+package api
+
+import (
+	"log/slog"
+	"net/http"
+	"strings"
+	"sync"
+	"time"
+)
+
+// In demo mode the database is reset to the sample data every DEMO_RESET_MINUTES. Requests hold a read
+// lock while they run and the reset takes the write lock, so no request sees half-dropped tables.
+// Long-lived streams (WebSocket, SSE, long polling) are left out, otherwise the reset would wait for
+// them forever; they reconnect or retry on their own. The demo accounts keep their IDs, so sessions
+// stay valid; clients get DEMO_RESET and reload their data.
+
+type demoState struct {
+	gate sync.RWMutex // read: a running request, write: the reset
+
+	mu        sync.Mutex
+	nextReset time.Time
+}
+
+func isLongLivedRequest(r *http.Request) bool {
+	p := r.URL.Path
+	return strings.HasPrefix(p, "/api/v1/ws") || p == "/api/v1/events" ||
+		(strings.HasPrefix(p, "/api/v1/docs/") && strings.HasSuffix(p, "/changes"))
+}
+
+func (s *Server) demoResetGate(next http.Handler) http.Handler {
+	if !s.cfg.DemoMode {
+		return next
+	}
+	return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		if isLongLivedRequest(r) || !strings.HasPrefix(r.URL.Path, "/api/") {
+			next.ServeHTTP(w, r)
+			return
+		}
+		s.demo.gate.RLock()
+		defer s.demo.gate.RUnlock()
+		next.ServeHTTP(w, r)
+	})
+}
+
+// demoNextReset is zero when no periodic reset runs.
+func (s *Server) demoNextReset() time.Time {
+	s.demo.mu.Lock()
+	defer s.demo.mu.Unlock()
+	return s.demo.nextReset
+}
+
+func (s *Server) setDemoNextReset(t time.Time) {
+	s.demo.mu.Lock()
+	s.demo.nextReset = t
+	s.demo.mu.Unlock()
+}
+
+func (s *Server) resetDemo() {
+	s.demo.gate.Lock()
+	err := s.store.ResetAndSeedDemo()
+	s.demo.gate.Unlock()
+	if err != nil {
+		slog.Error("Demo-Reset fehlgeschlagen", "error", err)
+		return
+	}
+	s.wsHub.BroadcastSystem("DEMO_RESET")
+}
+
+func (s *Server) startDemoResetTicker() {
+	interval := time.Duration(s.cfg.DemoResetMinutes) * time.Minute
+	for {
+		next := time.Now().Add(interval)
+		s.setDemoNextReset(next)
+		time.Sleep(time.Until(next))
+		s.resetDemo()
+	}
+}