Add purchase date picker and sells feature

- Purchase form now includes a date picker (defaults to today)
- New Sell model, CRUD endpoints (/sells), and stats integration
- AddSell and SellList components added to dashboard
- Portfolio chart updated to reflect sells over time

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

btc-portfolio/backend/app/models.py

@@ -13,6 +13,7 @@ class User(Base):
     is_admin = Column(Boolean, default=False, nullable=False, server_default='0')
 
     purchases = relationship("Purchase", back_populates="owner", cascade="all, delete")
+    sells = relationship("Sell", back_populates="owner", cascade="all, delete")
 
 
 class Purchase(Base):
@@ -27,6 +28,18 @@ class Purchase(Base):
     owner = relationship("User", back_populates="purchases")
 
 
+class Sell(Base):
+    __tablename__ = "sells"
+
+    id = Column(Integer, primary_key=True, index=True)
+    btc_amount = Column(Float, nullable=False)
+    price_eur = Column(Float, nullable=False)
+    created_at = Column(DateTime, default=datetime.utcnow)
+    user_id = Column(Integer, ForeignKey("users.id"), nullable=False)
+
+    owner = relationship("User", back_populates="sells")
+
+
 class OHLCCandle(Base):
     __tablename__ = "ohlc_candles"