Let a new user set up their hotel #82

Blocked by: #79, #80

Problem

Anyone can sign up with Google, but a new user has no hotel, so there's nothing for them to see. Today a hotel, its city and its room types can only be created by calling the API by hand, which a hotel manager can't do.

Goal

After signing up, a user without a hotel sets one up in a few steps and lands in the app as that hotel's first user.

Scope

• An onboarding endpoint (e.g. POST /onboarding/hotel) that, in one transaction, creates the hotel, its room type and the link between the signed-in user and the hotel.
• The details asked for:
  • Hotel name and location (city).
  • Total rooms.
  • One room type ("Standard") with its room count and normal nightly rate.
• City: pick an existing city, or create a new one from a search, filling in timezone, coordinates and nearest airport automatically.
• GET /auth/me says whether the user still needs to set up a hotel, so the frontend knows where to send them.
• After setup, start the first events and weather backfill for the new hotel.
• Not in scope: the onboarding screens (frontend), inviting other users to a hotel, more than one room type, and trials or billing.

Considerations

• A city's timezone and coordinates drive which nights and which events belong to the hotel; getting them wrong silently breaks recommendations. Fill them from a geocoding lookup rather than asking the user to type them.
• Two people from the same hotel could each create it. Until invites exist, the second person should contact us to be linked rather than create a duplicate.
• Validate the room count against total rooms and require a positive rate.
• Decide whether a user can set up a second hotel; one is enough for now.

Acceptance criteria

• A newly signed-up user can create a hotel and is linked to it as its first user.
• If any step fails, nothing is saved.
• GET /auth/me shows that setup is needed before, and not after.
• The new hotel's city has a timezone and coordinates without the user typing them.
• The first events and weather backfill starts for the new hotel.
• Tests cover success, validation errors and the rollback.