Apply demo-activity-bot: Bruno, the synthetic public-walk generator

Adds a background demo user (`bruno`) plus two pg-boss jobs that generate
public trekking routes and activities in inner Berlin and prune them
after 14 days. Disabled by default everywhere; flip `DEMO_BOT_ENABLED`
only in prod.

- Schema: `synthetic` boolean on routes + activities
- `ensureDemoUser()` idempotent insert + badge on /users/bruno
- Generation gate: 07-21 Berlin local, p=0.12, 40-route cap in 14d
- Initial 4-walk backfill on first enable; retention via daily prune
- Prometheus gauges `demo_bot_synthetic_routes_total` / `_activities_total`
- Unit + DB-gated integration + E2E tests

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

packages/i18n/src/locales/de.ts

@@ -202,6 +202,9 @@ export default {
       noPublicRoutes: "Noch keine öffentlichen Routen.",
       noPublicActivities: "Noch keine öffentlichen Aktivitäten.",
     },
+    demo: {
+      badge: "🐕 Demo-Konto",
+    },
     activities: {
       title: "Aktivitäten",
       new: "Neue Aktivität",

packages/i18n/src/locales/en.ts

@@ -202,6 +202,9 @@ export default {
       noPublicRoutes: "No public routes yet.",
       noPublicActivities: "No public activities yet.",
     },
+    demo: {
+      badge: "🐕 Demo account",
+    },
     activities: {
       title: "Activities",
       new: "New Activity",