Add support for pokemon evolutions in the tracker. When a pokemon evolves during a Nuzlocke run, the tracker should reflect the new species while preserving the encounter history (original catch route, catch level, nickname, etc.).
## Implementation
- **Data model**: `Evolution` table with from/to pokemon, trigger, level, item, and condition fields. `Encounter.current_pokemon_id` tracks evolved species separately from original.
- **Seed data**: Evolution chains fetched from PokeAPI submodule data, with an overrides file for manual corrections.
- **API**: `GET /pokemon/{id}/evolutions` returns available evolutions. `PATCH /encounters/{id}` accepts `current_pokemon_id` to record an evolution.
- **Frontend**: "Evolve" button in StatusChangeModal shows available evolutions with trigger details. PokemonCard displays the current (evolved) species with "Originally: {name}" label.
