feat(import): use a dedicated rejection reason for VIN conflicts instead of `:invalid_fields`

[JakobLichterfeld]

Context

Follow-up to #5519. The TeslaFi import quarantines a row when its VIN differs from the
import car while the vehicle ID still matches (classify_vehicle/5 in
lib/teslamate/import.ex). This handles TeslaFi variants that omit VINs and rely on the
configured vehicle-ID fallback.

Problem

The quarantined row is currently recorded as :invalid_fields, ["vin"], so the import
page shows "invalid values for vin". That is misleading: the VIN is a perfectly valid
value — it just belongs to a different vehicle than the one the import run identified.

Suggested change

• Add a dedicated reason such as :vehicle_conflict to the Ecto.Enum values in
  lib/teslamate/import/rejection.ex and use it in classify_vehicle/5.
• Add a matching rejection_reason/1 clause in
  lib/teslamate_web/live/import_live/index.ex with a gettext string along the lines of
  "row belongs to a different vehicle", and run the gettext extract/merge.

No migration is needed — the reason column is a plain :string.

🤖Issue drafted with Claude Code (Fable 5 high) — sponsored by Claude for Open Source

[JakobLichterfeld]

double check before working on it: #5531 reverted #5519