Feature: add Mistral EU regional inference backend

Add Mistral as an administrator-selectable AI backend for future CRM work.

Why:
- Mistral's EU regional endpoint (https://api.eu.mistral.ai) keeps inference in EU/EFTA data centres.
- It is a stronger fit than a China-hosted provider for CRM prompts containing lead/contact data.

Acceptance criteria:
- Add Mistral to the persisted web and mobile backend selectors.
- Route requests to the EU endpoint explicitly; do not use the global endpoint.
- Make model availability/configuration explicit and test the factory path.
- Document the provider in the processor register, privacy notice, and Art. 30 record.
- Before production enablement, confirm the commercial DPA, data-training/retention controls, subprocessors, and any remaining transfer evidence.

Reference: Mistral regional inference documentation and DPA, reviewed 2026-07-23.

No files or tests are named. Start by locating the persisted web and mobile backend selectors and the provider factory, then find the processor register, privacy notice, and Art. 30 record. Done means the EU endpoint, explicit model configuration, and factory path are tested, documentation is updated, and the listed DPA, retention, subprocessor, and transfer checks are confirmed.

Written by the indexing model from the issue text.