Add Pinwheel Clause support for zone-based encounters in route groups

Allows each sub-zone within a route group to have its own independent
encounter when the Pinwheel Clause rule is enabled (default on), instead
of the entire group sharing a single encounter.

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

backend/src/app/alembic/versions/a1b2c3d4e5f7_add_pinwheel_clause_support.py

@@ -0,0 +1,29 @@
+"""add pinwheel clause support
+
+Revision ID: a1b2c3d4e5f7
+Revises: f6a7b8c9d0e1
+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 = 'a1b2c3d4e5f7'
+down_revision: Union[str, Sequence[str], None] = 'f6a7b8c9d0e1'
+branch_labels: Union[str, Sequence[str], None] = None
+depends_on: Union[str, Sequence[str], None] = None
+
+
+def upgrade() -> None:
+    op.add_column(
+        'routes',
+        sa.Column('pinwheel_zone', sa.SmallInteger(), nullable=True),
+    )
+
+
+def downgrade() -> None:
+    op.drop_column('routes', 'pinwheel_zone')