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

backend/app/core/config.py

@@ -25,6 +25,12 @@ class Settings(BaseSettings):
     # Comma-separated list of origins allowed to call the API (frontend dev server, etc.)
     cors_origins: str = "*"
 
+    # CoinGecko Demo API key (free, no card required — https://www.coingecko.com/en/api/pricing)
+    # for services/crypto_service.py's price lookups. Empty by default; the
+    # Crypto tab's endpoints 400 with a clear message until this is set,
+    # rather than silently hitting CoinGecko's much stingier keyless tier.
+    coingecko_api_key: str = ""
+
     @property
     def database_url(self) -> str:
         return (