Phase 1.2: Create Exchange model

Add Exchange model with:
- user_id / user relationship
- slot_start / slot_end (datetime)
- direction (TradeDirection enum: buy/sell)
- eur_amount (int, cents), sats_amount (int)
- market_price_eur, agreed_price_eur (float)
- premium_percentage (int, snapshot)
- status (ExchangeStatus enum)
- created_at, cancelled_at, completed_at timestamps
- unique constraint on slot_start

backend/models.py

@@ -348,3 +348,56 @@ class PriceHistory(Base):
     created_at: Mapped[datetime] = mapped_column(
         DateTime(timezone=True), default=lambda: datetime.now(UTC)
     )
+
+
+class Exchange(Base):
+    """Bitcoin exchange trades booked by users."""
+
+    __tablename__ = "exchanges"
+    __table_args__ = (UniqueConstraint("slot_start", name="uq_exchange_slot_start"),)
+
+    id: Mapped[int] = mapped_column(Integer, primary_key=True, autoincrement=True)
+    user_id: Mapped[int] = mapped_column(
+        Integer, ForeignKey("users.id"), nullable=False, index=True
+    )
+    user: Mapped[User] = relationship("User", foreign_keys=[user_id], lazy="joined")
+
+    # Slot timing
+    slot_start: Mapped[datetime] = mapped_column(
+        DateTime(timezone=True), nullable=False, index=True
+    )
+    slot_end: Mapped[datetime] = mapped_column(DateTime(timezone=True), nullable=False)
+
+    # Trade details
+    direction: Mapped[TradeDirection] = mapped_column(
+        Enum(TradeDirection), nullable=False
+    )
+    eur_amount: Mapped[int] = mapped_column(Integer, nullable=False)  # EUR cents
+    sats_amount: Mapped[int] = mapped_column(Integer, nullable=False)  # Satoshis
+
+    # Price information (snapshot at booking time)
+    market_price_eur: Mapped[float] = mapped_column(
+        Float, nullable=False
+    )  # EUR per BTC
+    agreed_price_eur: Mapped[float] = mapped_column(
+        Float, nullable=False
+    )  # EUR per BTC with premium
+    premium_percentage: Mapped[int] = mapped_column(
+        Integer, nullable=False
+    )  # e.g. 5 for 5%
+
+    # Status
+    status: Mapped[ExchangeStatus] = mapped_column(
+        Enum(ExchangeStatus), nullable=False, default=ExchangeStatus.BOOKED
+    )
+
+    # Timestamps
+    created_at: Mapped[datetime] = mapped_column(
+        DateTime(timezone=True), default=lambda: datetime.now(UTC)
+    )
+    cancelled_at: Mapped[datetime | None] = mapped_column(
+        DateTime(timezone=True), nullable=True
+    )
+    completed_at: Mapped[datetime | None] = mapped_column(
+        DateTime(timezone=True), nullable=True
+    )