## Description of the issue/feature this PR addresses:
Turkish accounting applies different exchange rates depending on whether
the company is collecting foreign currency or paying it out. TCMB publishes
both buying and selling rates daily but `currency_rate_live` only keeps
the average, so accountants currently correct the rate by hand on every
invoice and bill.

## Current behavior before PR:
The TCMB daily update stores only the averaged rate on `res.currency.rate`.
Invoices and bills use that same rate, and users have to adjust it
manually per document.

## Desired behavior after PR is merged:
- Both the buying and selling rates are stored on `res.currency.rate`
  next to the average.
- Customer invoices and vendor bills get a Rate Type field, defaulting
  to "buying" for invoices and vendor refunds and to "selling" for bills
  and customer refunds. The user can override before posting.
- The selected rate is written to `invoice_currency_rate`. The lookup
  uses the rate dated the document's date, or the most recent rate
  before it when no rate exists for that exact date.
- Only TR companies with TRY as base currency are affected.

task-5017817
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