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/pokemon.py

@@ -0,0 +1,23 @@
+from sqlalchemy import SmallInteger, String
+from sqlalchemy.dialects.postgresql import ARRAY
+from sqlalchemy.orm import Mapped, mapped_column, relationship
+
+from app.core.database import Base
+
+
+class Pokemon(Base):
+    __tablename__ = "pokemon"
+
+    id: Mapped[int] = mapped_column(primary_key=True)
+    national_dex: Mapped[int] = mapped_column(SmallInteger, unique=True)
+    name: Mapped[str] = mapped_column(String(50))
+    types: Mapped[list[str]] = mapped_column(ARRAY(String(20)))
+    sprite_url: Mapped[str | None] = mapped_column(String(500))
+
+    route_encounters: Mapped[list["RouteEncounter"]] = relationship(
+        back_populates="pokemon"
+    )
+    encounters: Mapped[list["Encounter"]] = relationship(back_populates="pokemon")
+
+    def __repr__(self) -> str:
+        return f"<Pokemon(id={self.id}, name='{self.name}', dex={self.national_dex})>"