feat: persist shared demo routes in PostgreSQL

[andss]

Routes currently live in browser localStorage. This adds PostgreSQL persistence and a GraphQL contract for creating and listing routes for the existing shared demo account (ola-normann).

• Add an additive migration for routes and ordered Venue references, with stable IDs and ownership.
• Validate names, stop counts and existing Venue UUIDs; save each route in one transaction.
• Add createRoute(input: CreateRouteInput!): SavedRoute! and paginated myRoutes, composed in a separate GraphQL module.
• Document the contract and the frontend integration for Kasper.

Validation: backend typecheck and build pass; all 65 backend tests pass, including 8 new route tests covering migration reversal, ordering, owner filtering, pagination, constraints, rollback and API recreation. Targeted Prettier and git diff checks pass.

Part of #23. This draft is ready for discussion of the route contract with Aleks and Kasper. The current frontend still uses localStorage. After review/merge, Aleks coordinates the VM migration and backend deployment, followed by frontend integration. All visitors share the demo account; individual-user authentication and private routes are later work. No live database changes have been made.