feat: add a Crypto tab with live CoinGecko prices synced into Net Worth

Track individual coins (identity + quantity) in a dedicated tab, backed by
CoinGecko's Demo API (AURUM_COINGECKO_API_KEY). A holding is just another
Asset under the hood (asset_class=crypto) with its value written as a
regular AssetValuation — the existing Net Worth aggregation needs zero
changes to pick it up.

Prices never refresh in the background (no scheduler in this stack, same
reasoning as recurring transactions): a "Refresh prices" button forces a
sync, and GET /crypto/holdings lazily re-syncs at most once every 24h.
CoinGecko outages leave existing values untouched (error_key: "unreachable")
instead of failing the request. Backup/restore now carries CryptoHolding
rows too, so a restore doesn't silently strip a holding's coin identity.

Backend: 75/75 tests passing (9 new), including a real identity-map
staleness bug the tests caught in the raw-Core-upsert + ORM re-read pattern,
fixed via populate_existing=True.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01855PiW5Wd8n4aRW4n9kByY

frontend/src/App.tsx

@@ -10,6 +10,7 @@ import { AdvicePage } from "@/pages/AdvicePage";
 import { BudgetPage } from "@/pages/BudgetPage";
 import { CashFlowPage } from "@/pages/CashFlowPage";
 import { CategoriesPage } from "@/pages/CategoriesPage";
+import { CryptoPage } from "@/pages/CryptoPage";
 import { CsvImportPage } from "@/pages/CsvImportPage";
 import { DashboardPage } from "@/pages/DashboardPage";
 import { GoalsPage } from "@/pages/GoalsPage";
@@ -46,6 +47,7 @@ export default function App() {
           <Routes>
             <Route path="/" element={<DashboardPage />} />
             <Route path="/net-worth" element={<NetWorthPage />} />
+            <Route path="/crypto" element={<CryptoPage />} />
             <Route path="/roi" element={<RoiPage />} />
             <Route path="/accounts" element={<AccountsPage />} />
             <Route path="/categories" element={<CategoriesPage />} />