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/routes/purchases.py

@@ -1,7 +1,7 @@
 from fastapi import APIRouter, Depends, HTTPException, status
 from sqlalchemy.orm import Session
 from pydantic import BaseModel, Field
-from typing import List
+from typing import List, Optional
 from datetime import datetime
 
 from ..database import get_db
@@ -14,6 +14,7 @@ router = APIRouter()
 class PurchaseCreate(BaseModel):
     amount_eur: float = Field(gt=0, le=10_000_000)
     price_eur: float = Field(gt=0, le=10_000_000)
+    created_at: Optional[datetime] = None
 
 
 class PurchaseUpdate(BaseModel):
@@ -54,6 +55,7 @@ def add_purchase(
     purchase = models.Purchase(
         amount_eur=purchase_in.amount_eur,
         price_eur=purchase_in.price_eur,
+        created_at=purchase_in.created_at or datetime.utcnow(),
         user_id=current_user.id,
     )
     db.add(purchase)