Phase 003 Execution

Scope

Added VehiclesController with index, show, new, create, edit, update, and destroy actions.
Added resources :vehicles and changed the root route to vehicles#index.
Added plain ERB views for vehicle index, detail, new, edit, and the shared form.
Added query-param filtering on the vehicle index:
- default shows active vehicles
- status=inactive shows inactive vehicles
- status=all shows all vehicles
Implemented destroy as a deactivate action that sets active to false.
Displayed maintenance schedules, fuel history, and maintenance history on vehicles#show.
Ordered fuel and maintenance history newest first.
Updated home specs for the new root behavior.
Added vehicle request specs for successful and invalid create/update paths.
Added licence plate normalization to uppercase before validation.

Vehicles are not hard-deleted because maintenance and fuel history must be preserved.
The UI labels the destructive route as Deactivate to match actual behavior.
The vehicle show page is display-only for related schedules and histories in this phase.
Event creation remains reserved for Phase 004.
Maintenance schedule CRUD remains reserved for Phase 006.
Licence plates are standardized to uppercase before validation and storage.

Passed: bundle exec rspec spec/requests/vehicles_spec.rb
Passed: bundle exec rspec
Passed for touched Phase 003 Ruby files: bin/rubocop app/controllers/vehicles_controller.rb spec/requests/vehicles_spec.rb spec/requests/home_spec.rb spec/system/home_spec.rb
Full bin/rubocop was not clean because of pre-existing unrelated Gemfile single-quote offenses.