feat(demo): add demo manifest, Dutch demo entry, permanent badge and About page

Adds GET /api/v1/demo/manifest as a single source of truth for the demo's
fictional org identity (Northstar Mobility -- surfacing the project's
already-locked tenant name), synthetic-data/reset state, and live scenario
readiness. Rewrites the login screen in Dutch with an honest, no-password
demo entry and a guided-demo entry point, replaces the loud full-width
demo banner with a subtle badge + popover, and adds a compact About page
explaining what's real vs. synthetic vs. not yet connected.

backend/app/schemas.py

@@ -197,6 +197,40 @@ class IntegrationStatusOut(BaseModel):
     mcp_hub: McpHubIntegrationStatus
 
 
+class DemoScenarioOut(BaseModel):
+    id: str
+    title: str
+    operational_problem: str
+    estimated_minutes: int
+    required_roles: list[Role]
+    start_path: str
+    demonstrates: str
+    ready: bool
+    blocked_reason: str | None = None
+
+
+class DemoIntegrationSummaryOut(BaseModel):
+    key: Literal["n8n", "ragcore", "mcp_hub"]
+    label: str
+    status_label: str
+    detail: str
+
+
+class DemoManifestOut(BaseModel):
+    demo_mode: bool
+    organization_name: str
+    organization_description: str
+    timezone: str
+    synthetic_data: bool
+    allow_reset: bool
+    last_reset_at: datetime | None
+    anchor_date: str | None
+    guide_available: bool
+    required_roles: list[Role]
+    scenarios: list[DemoScenarioOut]
+    integrations: list[DemoIntegrationSummaryOut]
+
+
 class VehicleDetailOut(VehicleOut):
     bookings: list[BookingSummaryOut] = Field(default_factory=list)
     inspections: list[InspectionOut] = Field(default_factory=list)