Migrate geographies to API v2 alpha

[anth-volk]

Summary

Migrate the geography/region functionality in app v2 to use the new API v2 alpha regions endpoint, removing user-specific geography associations in favor of system-level reference data.

Current State

• App v2 uses UserGeographicStore with user associations (userId, custom label)
• UserGeographyPopulation type ties geographies to specific users
• Geography data is user-scoped with CRUD operations per user

Target State

• Fetch regions from API v2 alpha /regions/ endpoint (no user context)
• Regions are system reference data with standardized labels
• Remove user-geography association pattern entirely

Tasks

• Update app to fetch regions from new /regions/ endpoint
• Remove UserGeographicStore and user-scoped geography APIs
• Update UserGeographyPopulation or replace with simpler Region type
• Remove custom geography naming/labeling features
• Update geography selection UI to use system regions
• Update any components that depend on user-geography associations

Dependencies

This work depends on:

• policyengine.py PR Hardcode from 2024 to 2025 #231: Add ability to run impacts over subnational regions
• policyengine-api-v2-alpha PR Fix the Loading (new/existing) Population feature in Simulation flow #82: Add regions support for geographic analysis

Related

• Branch: feat/migrate-geographies-to-v2