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>

pnpm-lock.yaml

@@ -266,6 +266,9 @@ importers:
       react-router:
         specifier: 'catalog:'
         version: 7.14.1(react-dom@19.2.5(react@19.2.5))(react@19.2.5)
+      zod:
+        specifier: ^3.25.0
+        version: 3.25.76
     devDependencies:
       '@react-router/dev':
         specifier: 'catalog:'