Add nullable region field to evolutions for regional form filtering

Regional evolutions (e.g., Pikachu → Alolan Raichu) only occur in specific
regions. This adds a nullable region column so the app can filter evolutions
by the game's region. When a regional evolution exists for a given trigger/item,
the non-regional counterpart is automatically hidden.

Full-stack: migration, model, schemas, API with region query param, seeder,
Go fetch tool, frontend types/API/hook/components, and admin form.

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

backend/src/app/models/evolution.py

@@ -15,6 +15,7 @@ class Evolution(Base):
     item: Mapped[str | None] = mapped_column(String(50))  # e.g. thunder-stone
     held_item: Mapped[str | None] = mapped_column(String(50))
     condition: Mapped[str | None] = mapped_column(String(200))  # catch-all for other conditions
+    region: Mapped[str | None] = mapped_column(String(30))
 
     from_pokemon: Mapped["Pokemon"] = relationship(foreign_keys=[from_pokemon_id])
     to_pokemon: Mapped["Pokemon"] = relationship(foreign_keys=[to_pokemon_id])