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

README.md

@@ -63,6 +63,9 @@ Log income, expenses, and transfers across as many accounts as you want, organiz
 ### 📈 Net Worth Engine
 A live net worth timeline (30 days to all-time) aggregating cash and every manually tracked asset — investments, crypto, real estate, vehicles, precious metals — into one number, with a full breakdown by asset class, by how each asset behaves (income / neutral / drain), and by risk level.
 
+### 🪙 Crypto Tracker
+A dedicated tab for the coins you actually hold — add a token and a quantity, and Aurum keeps its value current against live CoinGecko prices, no separate portfolio tracker needed. Refresh on demand with one button, or let it auto-refresh once a day; either way it plugs straight into the Net Worth engine above as just another tracked asset.
+
 ### 🎯 Budgets, Goals & Recurring Payments
 Set a monthly limit per category and watch progress bars fill up. Track savings goals with a running contribution log. Register recurring bills and post them with one click when they're due — nothing runs automatically in the background.