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/models/__init__.py

@@ -5,6 +5,7 @@ from app.models.encounter import Encounter
 from app.models.evolution import Evolution
 from app.models.game import Game
 from app.models.genlocke import Genlocke, GenlockeLeg
+from app.models.genlocke_transfer import GenlockeTransfer
 from app.models.nuzlocke_run import NuzlockeRun
 from app.models.pokemon import Pokemon
 from app.models.route import Route
@@ -20,6 +21,7 @@ __all__ = [
     "Game",
     "Genlocke",
     "GenlockeLeg",
+    "GenlockeTransfer",
     "NuzlockeRun",
     "Pokemon",
     "Route",

backend/src/app/models/genlocke_transfer.py

@@ -0,0 +1,32 @@
+from datetime import datetime
+
+from sqlalchemy import DateTime, ForeignKey, SmallInteger, UniqueConstraint, func
+from sqlalchemy.orm import Mapped, mapped_column
+
+from app.core.database import Base
+
+
+class GenlockeTransfer(Base):
+    __tablename__ = "genlocke_transfers"
+    __table_args__ = (
+        UniqueConstraint("target_encounter_id", name="uq_genlocke_transfers_target"),
+    )
+
+    id: Mapped[int] = mapped_column(primary_key=True)
+    genlocke_id: Mapped[int] = mapped_column(
+        ForeignKey("genlockes.id", ondelete="CASCADE"), index=True
+    )
+    source_encounter_id: Mapped[int] = mapped_column(
+        ForeignKey("encounters.id"), index=True
+    )
+    target_encounter_id: Mapped[int] = mapped_column(
+        ForeignKey("encounters.id"), unique=True
+    )
+    source_leg_order: Mapped[int] = mapped_column(SmallInteger)
+    target_leg_order: Mapped[int] = mapped_column(SmallInteger)
+    created_at: Mapped[datetime] = mapped_column(
+        DateTime(timezone=True), server_default=func.now()
+    )
+
+    def __repr__(self) -> str:
+        return f"<GenlockeTransfer(id={self.id}, source={self.source_encounter_id}, target={self.target_encounter_id})>"