Add country field and support more fields in person CSV import/export

[NicoPennec]

Problem

• Person has no country, which the carbon-footprint plugin needs to estimate travel emissions.
• CSV person import/export only covers name, email, phone, role and contract type, so studios cannot bulk-manage country, position, seniority, daily salary, studio and departments.
• ISO 3166-1 country validation would otherwise be duplicated across the model, the API, the CSV import and the SAML SSO path.

Solution

• Add a nullable ISO 3166-1 alpha-2 country column on Person, validated and uppercased, settable via POST/PUT and SAML SSO, and kept out of the minimal (non-manager) payload.
• Import/export country, position, seniority, daily salary, studio and departments; typed columns accept either the human label or the stored code, studio/department names match exactly (case-sensitive), and invalid values fail the row with a 400.
• Factor the validation into a single normalize_country helper shared by the model validator (silently discards), the API guard (400) and the CSV import (fails the row).
• On SAML SSO, normalize the country to its canonical form so an unchanged value no longer re-triggers an update on every login; an empty value clears it while a malformed assertion is ignored rather than wiping a stored code or breaking sign-in.
• Add a migration for the nullable country column.