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_encounter.py

@@ -0,0 +1,25 @@
+from sqlalchemy import ForeignKey, SmallInteger, String, UniqueConstraint
+from sqlalchemy.orm import Mapped, mapped_column, relationship
+
+from app.core.database import Base
+
+
+class RouteEncounter(Base):
+    __tablename__ = "route_encounters"
+    __table_args__ = (
+        UniqueConstraint(
+            "route_id", "pokemon_id", "encounter_method", name="uq_route_pokemon_method"
+        ),
+    )
+
+    id: Mapped[int] = mapped_column(primary_key=True)
+    route_id: Mapped[int] = mapped_column(ForeignKey("routes.id"), index=True)
+    pokemon_id: Mapped[int] = mapped_column(ForeignKey("pokemon.id"), index=True)
+    encounter_method: Mapped[str] = mapped_column(String(30))
+    encounter_rate: Mapped[int] = mapped_column(SmallInteger)
+
+    route: Mapped["Route"] = relationship(back_populates="route_encounters")
+    pokemon: Mapped["Pokemon"] = relationship(back_populates="route_encounters")
+
+    def __repr__(self) -> str:
+        return f"<RouteEncounter(route_id={self.route_id}, pokemon_id={self.pokemon_id}, method='{self.encounter_method}')>"