Add database schema with SQLAlchemy async + Alembic migrations

Set up PostgreSQL database layer with async SQLAlchemy 2.0 and asyncpg driver.
Implements 6 core tables (games, routes, pokemon, route_encounters, nuzlocke_runs,
encounters) with foreign keys, indexes, and an initial Alembic migration.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

backend/src/app/models/route.py

@@ -0,0 +1,22 @@
+from sqlalchemy import ForeignKey, SmallInteger, String
+from sqlalchemy.orm import Mapped, mapped_column, relationship
+
+from app.core.database import Base
+
+
+class Route(Base):
+    __tablename__ = "routes"
+
+    id: Mapped[int] = mapped_column(primary_key=True)
+    name: Mapped[str] = mapped_column(String(100))
+    game_id: Mapped[int] = mapped_column(ForeignKey("games.id"), index=True)
+    order: Mapped[int] = mapped_column(SmallInteger)
+
+    game: Mapped["Game"] = relationship(back_populates="routes")
+    route_encounters: Mapped[list["RouteEncounter"]] = relationship(
+        back_populates="route"
+    )
+    encounters: Mapped[list["Encounter"]] = relationship(back_populates="route")
+
+    def __repr__(self) -> str:
+        return f"<Route(id={self.id}, name='{self.name}')>"