Add users and link them to hotels #79

Problem

The API has no idea who is calling it. There's no users table, so we can't sign anyone in, limit what they see, or record who made a decision.

Goal

Users exist in the database, each with a Google identity, and each can be linked to the hotels they work for.

Scope

• A users table: Google account ID (unique), email, name, created at, last signed in at.
• A link table between users and hotels, so a hotel can have several users and a user can belong to more than one hotel later. A user with no hotel is allowed (they've just signed up).
• Models, repository and services through every layer, following the Widget slice.
• An Alembic migration that applies and rolls back.
• A command to link a user to a hotel by email (e.g. just link-user <email> <hotel_id>), until there's a way to do it in the app.
• Not in scope: passwords; sign-in is Google only.

Considerations

• Identify users by Google's account ID (sub), not their email: emails can change.
• Keep the link table simple (user, hotel, created at). Roles can be added later if needed.

Acceptance criteria

• The migration upgrades and downgrades cleanly.
• A user can be created, found by Google account ID, and linked to a hotel.
• Linking the same user to the same hotel twice doesn't create a duplicate.
• Tests cover the repository and service.