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>
Backend: PUT /purchases/{id} endpoint accepts updated amount_eur,
price_eur, and created_at. Frontend: inline edit row in PurchaseList
with date picker and number inputs, Save/Cancel actions.
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>