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/game.py

@@ -0,0 +1,22 @@
+from sqlalchemy import SmallInteger, String
+from sqlalchemy.orm import Mapped, mapped_column, relationship
+
+from app.core.database import Base
+
+
+class Game(Base):
+    __tablename__ = "games"
+
+    id: Mapped[int] = mapped_column(primary_key=True)
+    name: Mapped[str] = mapped_column(String(100))
+    slug: Mapped[str] = mapped_column(String(100), unique=True)
+    generation: Mapped[int] = mapped_column(SmallInteger)
+    region: Mapped[str] = mapped_column(String(50))
+    box_art_url: Mapped[str | None] = mapped_column(String(500))
+    release_year: Mapped[int | None] = mapped_column(SmallInteger)
+
+    routes: Mapped[list["Route"]] = relationship(back_populates="game")
+    runs: Mapped[list["NuzlockeRun"]] = relationship(back_populates="game")
+
+    def __repr__(self) -> str:
+        return f"<Game(id={self.id}, name='{self.name}')>"