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/api/pokemon.py

@@ -1,5 +1,5 @@
 from fastapi import APIRouter, Depends, HTTPException, Query
-from sqlalchemy import func, select
+from sqlalchemy import func, or_, select
 from sqlalchemy.ext.asyncio import AsyncSession
 from sqlalchemy.orm import joinedload, selectinload
 
@@ -121,18 +121,40 @@ async def get_pokemon(
 
 @router.get("/pokemon/{pokemon_id}/evolutions", response_model=list[EvolutionResponse])
 async def get_pokemon_evolutions(
-    pokemon_id: int, session: AsyncSession = Depends(get_session)
+    pokemon_id: int,
+    region: str | None = Query(None),
+    session: AsyncSession = Depends(get_session),
 ):
     pokemon = await session.get(Pokemon, pokemon_id)
     if pokemon is None:
         raise HTTPException(status_code=404, detail="Pokemon not found")
 
-    result = await session.execute(
+    query = (
         select(Evolution)
         .where(Evolution.from_pokemon_id == pokemon_id)
         .options(joinedload(Evolution.to_pokemon))
     )
-    return result.scalars().unique().all()
+    if region is not None:
+        query = query.where(
+            or_(Evolution.region.is_(None), Evolution.region == region)
+        )
+    result = await session.execute(query)
+    evolutions = result.scalars().unique().all()
+
+    if region is not None:
+        # Regional evolutions replace the non-regional one that shares the
+        # same trigger + item (e.g. Pikachu + thunder-stone → Alolan Raichu
+        # replaces Pikachu + thunder-stone → Raichu in Alola).
+        regional_keys = {
+            (e.trigger, e.item) for e in evolutions if e.region is not None
+        }
+        if regional_keys:
+            evolutions = [
+                e for e in evolutions
+                if e.region is not None or (e.trigger, e.item) not in regional_keys
+            ]
+
+    return evolutions
 
 
 @router.put("/pokemon/{pokemon_id}", response_model=PokemonResponse)