Epic 4: Migrate the backend from Express to Hono

Replace Express with Hono across the whole backend. Validation moves into the route definitions, the frontend gets a fully typed RPC client instead of a hand-rolled fetch wrapper, and Socket.IO gets replaced with server-sent events for the admin job progress. Express, Socket.IO and their middleware stack all get deleted at the end.

The existing backend integration tests are the safety net: they should pass against the new implementation before anything ships.