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/encounter.py

@@ -0,0 +1,29 @@
+from datetime import datetime
+
+from sqlalchemy import DateTime, ForeignKey, SmallInteger, String, func
+from sqlalchemy.orm import Mapped, mapped_column, relationship
+
+from app.core.database import Base
+
+
+class Encounter(Base):
+    __tablename__ = "encounters"
+
+    id: Mapped[int] = mapped_column(primary_key=True)
+    run_id: Mapped[int] = mapped_column(ForeignKey("nuzlocke_runs.id"), index=True)
+    route_id: Mapped[int] = mapped_column(ForeignKey("routes.id"), index=True)
+    pokemon_id: Mapped[int] = mapped_column(ForeignKey("pokemon.id"), index=True)
+    nickname: Mapped[str | None] = mapped_column(String(50))
+    status: Mapped[str] = mapped_column(String(20))  # caught, fainted, missed
+    catch_level: Mapped[int | None] = mapped_column(SmallInteger)
+    faint_level: Mapped[int | None] = mapped_column(SmallInteger)
+    caught_at: Mapped[datetime] = mapped_column(
+        DateTime(timezone=True), server_default=func.now()
+    )
+
+    run: Mapped["NuzlockeRun"] = relationship(back_populates="encounters")
+    route: Mapped["Route"] = relationship(back_populates="encounters")
+    pokemon: Mapped["Pokemon"] = relationship(back_populates="encounters")
+
+    def __repr__(self) -> str:
+        return f"<Encounter(id={self.id}, pokemon_id={self.pokemon_id}, status='{self.status}')>"