Turn Job Preferences Location UK Area to Geometry (#8438)

## Trello card URL

- Sentry errors:
  - https://teaching-vacancies.sentry.io/issues/7194829661
  - https://teaching-vacancies.sentry.io/issues/7194436126
 
## Changes in this PR:

The polygon type was causing errors when the incoming area was a
multipolygon.

Resolving it by converting it to the Geometry type, as done for the
Subscription UK area.

## Checklists:

### Data & Schema Changes

If this PR modifies data structures or validations, check the following:

- [ ] Adds/removes model validations
- [ ] Adds/removes database fields
- [ ] Modifies Vacancy enumerables (phases, working patterns, job roles,
key stages, etc.)

<details>
<summary>If any of the above options has changed then the author must
check/resolve all of the following...</summary>

### Integration Impact

Does this change affect any of these integrations?
- [ ] DfE Analytics platform
- [ ] Legacy imports mappings
- [ ] DWP Find a Job export mappings
- [ ] Publisher ATS API (may require mapping updates or API versioning)

### User Experience & Data Integrity

Could this change impact:
- [ ] Existing subscription alerts (will legacy subscription search
filters break?)
- [ ] Legacy vacancy copying (will copied vacancies fail new
validations?)
- [ ] In-progress drafts for Vacancies or Job Applications
</details>

Author: scruti

db/migrate/20260120090246_change_job_preferences_locations_uk_area_type.rb

@@ -0,0 +1,13 @@
+class ChangeJobPreferencesLocationsUkAreaType < ActiveRecord::Migration[8.0]
+  disable_ddl_transaction!
+
+  def change
+    add_column :job_preferences_locations, :new_uk_area, :geometry, srid: 27_700
+    add_index :job_preferences_locations, :new_uk_area, using: :gist, algorithm: :concurrently
+    remove_index :job_preferences_locations, :uk_area, algorithm: :concurrently
+    safety_assured do
+      remove_column :job_preferences_locations, :uk_area, :st_polygon, srid: 27_700
+      rename_column :job_preferences_locations, :new_uk_area, :uk_area
+    end
+  end
+end

db/schema.rb

@@ -10,7 +10,7 @@
 #
 # It's strongly recommended that you check this file into your version control system.
 
-ActiveRecord::Schema[8.0].define(version: 2026_01_15_140453) do
+ActiveRecord::Schema[8.0].define(version: 2026_01_20_090246) do
   # These are extensions that must be enabled in order to support this database
   enable_extension "btree_gist"
   enable_extension "citext"
@@ -331,7 +331,7 @@
     t.geography "area", limit: {srid: 4326, type: "geometry", geographic: true}, null: false
     t.datetime "created_at", null: false
     t.datetime "updated_at", null: false
-    t.geometry "uk_area", limit: {srid: 27700, type: "st_polygon"}
+    t.geometry "uk_area", limit: {srid: 27700, type: "geometry"}
     t.index ["area"], name: "index_job_preferences_locations_on_area", using: :gist
     t.index ["job_preferences_id"], name: "index_job_preferences_locations_on_job_preferences_id"
     t.index ["uk_area"], name: "index_job_preferences_locations_on_uk_area", using: :gist