Store manual rate overrides #86

Blocked by: #84

Problem

Staff sometimes set a booking's price by hand (discounts, fixes). Those aren't market prices, and if the model trains on them, its recommendations drift toward them.

Goal

Every manual rate override is stored and the nights and bookings it affected are marked, so training can leave them out.

Scope

• A table for overrides: booking reference, applied date, arrival, departure, rate from, rate to, rate plan, room type, pre- and post-override total.
• A parser for the Rate Override report.
• A way for other code to ask whether a booking night was overridden.
• Models, repository and services through every layer; idempotent upserts.
• An Alembic migration that applies and rolls back.

Acceptance criteria

• A sample file loads into the new table.
• Loading the same file twice doesn't add rows.
• Given a booking and night, the code can tell whether its rate was overridden.
• Tests cover parsing, upserts and the lookup.