Apply configurable-demo-persona: per-instance demo identity + voice

Adds DEMO_BOT_PERSONA env var (inline JSON or file:<path>) so
self-hosted instances can replace Bruno-in-Berlin with their own
demo account identity and content pools. No-op for trails.cool — the
built-in Bruno persona remains the default.

- DemoPersona type + Zod schema validation
- loadPersona() cached at boot; falls back to default on any failure
- ensureDemoUser throws DemoPersonaUsernameClashError when the
  persona username is already a real user; server declines to
  schedule demo jobs for that process
- isDemoUser flag moved from hardcoded "bruno" check to loader-supplied
  boolean computed from the running persona
- docs/demo-persona.md explains the schema + operator flow

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

apps/journal/package.json

@@ -35,7 +35,8 @@
     "prom-client": "^15.1.3",
     "react": "catalog:",
     "react-dom": "catalog:",
-    "react-router": "catalog:"
+    "react-router": "catalog:",
+    "zod": "^3.25.0"
   },
   "devDependencies": {
     "@react-router/dev": "catalog:",