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/alembic/versions/f6a7b8c9d0e1_add_region_to_evolutions.py

@@ -0,0 +1,29 @@
+"""add region to evolutions
+
+Revision ID: f6a7b8c9d0e1
+Revises: e5f6a7b8c9d0
+Create Date: 2026-02-07 12:00:00.000000
+
+"""
+from typing import Sequence, Union
+
+from alembic import op
+import sqlalchemy as sa
+
+
+# revision identifiers, used by Alembic.
+revision: str = 'f6a7b8c9d0e1'
+down_revision: Union[str, Sequence[str], None] = 'e5f6a7b8c9d0'
+branch_labels: Union[str, Sequence[str], None] = None
+depends_on: Union[str, Sequence[str], None] = None
+
+
+def upgrade() -> None:
+    op.add_column(
+        'evolutions',
+        sa.Column('region', sa.String(30), nullable=True),
+    )
+
+
+def downgrade() -> None:
+    op.drop_column('evolutions', 'region')