Add pokemon evolution support across the full stack

- Evolution model with trigger, level, item, and condition fields
- Encounter.current_pokemon_id tracks evolved species separately
- Alembic migration for evolutions table and current_pokemon_id column
- Seed pipeline loads evolution data with manual overrides
- GET /pokemon/{id}/evolutions and PATCH /encounters/{id} endpoints
- Evolve button in StatusChangeModal with evolution method details
- PokemonCard shows evolved species with "Originally" label

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

backend/src/app/models/__init__.py

@@ -1,4 +1,5 @@
 from app.models.encounter import Encounter
+from app.models.evolution import Evolution
 from app.models.game import Game
 from app.models.nuzlocke_run import NuzlockeRun
 from app.models.pokemon import Pokemon
@@ -7,6 +8,7 @@ from app.models.route_encounter import RouteEncounter
 
 __all__ = [
     "Encounter",
+    "Evolution",
     "Game",
     "NuzlockeRun",
     "Pokemon",