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/config/config.go

@@ -2,6 +2,7 @@ package config
 
 import (
 	"crypto/rand"
+	"strconv"
 	"encoding/hex"
 	"log/slog"
 	"os"
@@ -22,6 +23,8 @@ type Config struct {
 	LogLevel          string
 	Debug             bool
 	DemoMode          bool
+	// DemoResetMinutes: in demo mode the database is reset to the sample data this often (0 = never)
+	DemoResetMinutes int
 
 	// Optional SMTP environment variables (can also be overridden/configured in DB system_settings)
 	SMTPHost string
@@ -161,6 +164,7 @@ func Load() *Config {
 		LogLevel:          logLevel,
 		Debug:             debug,
 		DemoMode:          demoMode,
+		DemoResetMinutes:  demoResetMinutes(),
 		SMTPHost:          smtpHost,
 		SMTPPort:          smtpPort,
 		SMTPUser:          smtpUser,
@@ -197,3 +201,11 @@ func getEnv(key, fallback string) string {
 	}
 	return fallback
 }
+
+func demoResetMinutes() int {
+	n, err := strconv.Atoi(strings.TrimSpace(getEnv("DEMO_RESET_MINUTES", "20")))
+	if err != nil || n < 0 {
+		return 20
+	}
+	return n
+}