feat: nested geofences — area_manager access to all child areas of a region

[dadofsambonzuki]

Closes #111

Summary

Replaces the flat-list geofence model with spatial nesting. Geofencing a country-level area now grants access to all communities within it — no more manual ID syncing.

What changed

check_geofence() (src/service/area.rs)

• Direct ID match still works (backward compatible).
• New: if the target area is not directly listed, it checks whether the area's centroid falls inside any geofenced parent's polygon. If so, access is granted.

add_area handler (src/rpc/area/add_area.rs)

• Previously: any non-empty geofence blocked area creation entirely ("Cannot add new areas when your geofence is set").
• New: area managers can create new areas if the new area's centroid falls inside a geofenced parent region. Areas outside all geofenced parents are still rejected.

Shared helpers

• area_is_contained_in_geofence() — spatial check for existing areas (uses geometry centroid + polygon containment).
• geo_json_is_within_geofence() — spatial check for incoming GeoJSON (used by add_area before the area exists in the DB).
• inner_centroid_of_geometries() / point_is_inside_any() — reusable geometry utilities.

Tests (6 new/changed)

Test	What it verifies
check_geofence_allows_nested_child_area_within_geofenced_parent	Geofenced parent → accesses child by alias + numeric ID
geo_json_is_within_geofence_detects_containment	Inline geo_json containment check
area_manager_can_create_area_inside_geofenced_parent	add_area succeeds when within geofenced parent
area_manager_with_geofence_is_rejected_when_outside (renamed)	add_area still rejected when outside
area_manager_with_geofence_cannot_delete_unfenced_area (fixed)	Updated to use non-overlapping test areas

Backward compatibility

• Existing flat-list geofences continue working — the spatial check is additive.
• No schema changes.
• All 761 existing tests pass.

Example: UK area manager

Before:

geofence = [364, 53, 79, 93, 98, 100, ...]  # 43 IDs

After:

geofence = [364]  # UK country — auto-grants all communities inside