feat: add PriceHistory model for storing exchange price data

backend/models.py

@@ -376,3 +376,23 @@ class RandomNumberOutcome(Base):
     created_at: Mapped[datetime] = mapped_column(
         DateTime(timezone=True), default=lambda: datetime.now(UTC)
     )
+
+
+class PriceHistory(Base):
+    """Price history records from external exchanges."""
+
+    __tablename__ = "price_history"
+    __table_args__ = (
+        UniqueConstraint("source", "pair", "timestamp", name="uq_price_source_pair_ts"),
+    )
+
+    id: Mapped[int] = mapped_column(Integer, primary_key=True, autoincrement=True)
+    source: Mapped[str] = mapped_column(String(50), nullable=False, index=True)
+    pair: Mapped[str] = mapped_column(String(20), nullable=False)
+    price: Mapped[float] = mapped_column(Float, nullable=False)
+    timestamp: Mapped[datetime] = mapped_column(
+        DateTime(timezone=True), nullable=False, index=True
+    )
+    created_at: Mapped[datetime] = mapped_column(
+        DateTime(timezone=True), default=lambda: datetime.now(UTC)
+    )