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

.env.example

@@ -12,6 +12,12 @@ AURUM_POSTGRES_PORT=5432
 AURUM_CORS_ORIGINS=*
 AURUM_DEFAULT_CURRENCY=USD
 
+# --- Crypto tab (live prices via CoinGecko) ---
+# Free Demo API key, no card required: coingecko.com/en/api/pricing. Leave
+# blank and the Crypto tab's price sync just 400s with a message telling
+# you to set this — nothing else in the app is affected.
+AURUM_COINGECKO_API_KEY=
+
 # --- Basic auth (STRONGLY recommended if this instance is reachable from
 # anywhere other than your own machine) ---
 # Aurum has no login system of its own — it's built for one person