Add 1-year BTC daily price history chart

New GET /history endpoint fetches 365 days of BTC/EUR data from
CoinGecko, deduplicates by date, and joins the user's purchases.
BTCHistoryChart component renders the price line with orange dot
markers on purchase dates and a dashed cyan avg buy price line.
Tooltip shows purchase details on marked dates.

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

btc-portfolio/backend/app/main.py

@@ -2,7 +2,7 @@ from fastapi import FastAPI
 from fastapi.middleware.cors import CORSMiddleware
 
 from .database import engine, Base
-from .routes import users, purchases, stats
+from .routes import users, purchases, stats, history
 
 Base.metadata.create_all(bind=engine)
 
@@ -19,6 +19,7 @@ app.add_middleware(
 app.include_router(users.router)
 app.include_router(purchases.router)
 app.include_router(stats.router)
+app.include_router(history.router)
 
 
 @app.get("/")