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

docker-compose.yml

@@ -25,6 +25,9 @@ services:
       AURUM_POSTGRES_PORT: 5432
       AURUM_CORS_ORIGINS: ${AURUM_CORS_ORIGINS:-*}
       AURUM_DEFAULT_CURRENCY: ${AURUM_DEFAULT_CURRENCY:-USD}
+      # Free CoinGecko Demo key (coingecko.com/en/api/pricing) — the Crypto
+      # tab's price sync 400s with a clear message until this is set.
+      AURUM_COINGECKO_API_KEY: ${AURUM_COINGECKO_API_KEY:-}
     depends_on:
       db:
         condition: service_healthy