Add full-stack BTC portfolio web app

Multi-user FastAPI + React app with JWT auth, SQLite storage, and
CoinGecko price integration. Dockerized with docker-compose.

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

btc-portfolio/backend/app/models.py

@@ -0,0 +1,26 @@
+from sqlalchemy import Column, Integer, String, Float, ForeignKey, DateTime
+from sqlalchemy.orm import relationship
+from datetime import datetime
+from .database import Base
+
+
+class User(Base):
+    __tablename__ = "users"
+
+    id = Column(Integer, primary_key=True, index=True)
+    username = Column(String, unique=True, index=True, nullable=False)
+    password = Column(String, nullable=False)
+
+    purchases = relationship("Purchase", back_populates="owner", cascade="all, delete")
+
+
+class Purchase(Base):
+    __tablename__ = "purchases"
+
+    id = Column(Integer, primary_key=True, index=True)
+    amount_eur = 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="purchases")