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/hooks/useCrypto.ts

@@ -0,0 +1,59 @@
+import { useMutation, useQuery, useQueryClient } from "@tanstack/react-query";
+import {
+  createCryptoHolding,
+  deleteCryptoHolding,
+  fetchCryptoHoldings,
+  refreshCryptoPrices,
+  searchCryptoCoins,
+  updateCryptoHoldingQuantity,
+} from "@/api/crypto";
+import type { CryptoHoldingInput } from "@/types";
+
+function useInvalidateCrypto() {
+  const queryClient = useQueryClient();
+  return () => {
+    queryClient.invalidateQueries({ queryKey: ["crypto-holdings"] });
+    // A crypto holding is a normal Asset under the hood — Net Worth's own
+    // numbers change the moment one is added/edited/repriced/removed.
+    queryClient.invalidateQueries({ queryKey: ["net-worth-summary"] });
+  };
+}
+
+export function useCryptoHoldings() {
+  return useQuery({ queryKey: ["crypto-holdings"], queryFn: fetchCryptoHoldings });
+}
+
+export function useRefreshCryptoPrices() {
+  const invalidate = useInvalidateCrypto();
+  return useMutation({ mutationFn: refreshCryptoPrices, onSuccess: invalidate });
+}
+
+export function useCreateCryptoHolding() {
+  const invalidate = useInvalidateCrypto();
+  return useMutation({
+    mutationFn: (input: CryptoHoldingInput) => createCryptoHolding(input),
+    onSuccess: invalidate,
+  });
+}
+
+export function useUpdateCryptoHoldingQuantity() {
+  const invalidate = useInvalidateCrypto();
+  return useMutation({
+    mutationFn: ({ assetId, quantity }: { assetId: number; quantity: string }) =>
+      updateCryptoHoldingQuantity(assetId, quantity),
+    onSuccess: invalidate,
+  });
+}
+
+export function useDeleteCryptoHolding() {
+  const invalidate = useInvalidateCrypto();
+  return useMutation({
+    mutationFn: (assetId: number) => deleteCryptoHolding(assetId),
+    onSuccess: invalidate,
+  });
+}
+
+// Not a useQuery — this drives a search-as-you-type picker inside the "add
+// holding" modal, called imperatively (debounced) rather than kept in sync
+// with a query key.
+export { searchCryptoCoins };