Add genlocke transfer UI with transfer selection modal and backend support

When advancing to the next genlocke leg, users can now select surviving
Pokemon to transfer. Transferred Pokemon are bred down to their base
evolutionary form and appear as level-1 egg encounters in the next leg.
A GenlockeTransfer record links source and target encounters for lineage tracking.

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

backend/src/app/alembic/versions/e5f6a7b9c0d1_add_genlocke_transfers_table.py

@@ -0,0 +1,36 @@
+"""add genlocke_transfers table
+
+Revision ID: e5f6a7b9c0d1
+Revises: d4e5f6a7b9c0
+Create Date: 2026-02-09 22:00:00.000000
+
+"""
+from typing import Sequence, Union
+
+from alembic import op
+import sqlalchemy as sa
+
+
+# revision identifiers, used by Alembic.
+revision: str = 'e5f6a7b9c0d1'
+down_revision: Union[str, Sequence[str], None] = 'd4e5f6a7b9c0'
+branch_labels: Union[str, Sequence[str], None] = None
+depends_on: Union[str, Sequence[str], None] = None
+
+
+def upgrade() -> None:
+    op.create_table(
+        'genlocke_transfers',
+        sa.Column('id', sa.Integer(), primary_key=True),
+        sa.Column('genlocke_id', sa.Integer(), sa.ForeignKey('genlockes.id', ondelete='CASCADE'), nullable=False, index=True),
+        sa.Column('source_encounter_id', sa.Integer(), sa.ForeignKey('encounters.id'), nullable=False, index=True),
+        sa.Column('target_encounter_id', sa.Integer(), sa.ForeignKey('encounters.id'), nullable=False, unique=True),
+        sa.Column('source_leg_order', sa.SmallInteger(), nullable=False),
+        sa.Column('target_leg_order', sa.SmallInteger(), nullable=False),
+        sa.Column('created_at', sa.DateTime(timezone=True), server_default=sa.func.now(), nullable=False),
+        sa.UniqueConstraint('target_encounter_id', name='uq_genlocke_transfers_target'),
+    )
+
+
+def downgrade() -> None:
+    op.drop_table('genlocke_transfers')