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/main.py

@@ -3,7 +3,7 @@ from fastapi.middleware.cors import CORSMiddleware
 from sqlalchemy import text
 
 from .database import engine, Base, SessionLocal
-from .routes import users, purchases, stats, history, admin, candles
+from .routes import users, purchases, stats, history, admin, candles, sells
 
 Base.metadata.create_all(bind=engine)
 
@@ -23,6 +23,7 @@ app.include_router(stats.router)
 app.include_router(history.router)
 app.include_router(admin.router, prefix="/admin")
 app.include_router(candles.router)
+app.include_router(sells.router)
 
 
 @app.on_event("startup")