Fix ambiguous foreign keys on Pokemon.encounters relationship

Encounter has two FKs to pokemon (pokemon_id and current_pokemon_id),
so the reverse relationship needs an explicit foreign_keys argument.

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

backend/src/app/models/pokemon.py

@@ -17,7 +17,9 @@ class Pokemon(Base):
     route_encounters: Mapped[list["RouteEncounter"]] = relationship(
         back_populates="pokemon"
     )
-    encounters: Mapped[list["Encounter"]] = relationship(back_populates="pokemon")
+    encounters: Mapped[list["Encounter"]] = relationship(
+        foreign_keys="[Encounter.pokemon_id]", back_populates="pokemon"
+    )
 
     def __repr__(self) -> str:
         return f"<Pokemon(id={self.id}, name='{self.name}', dex={self.national_dex})>"