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/schemas/backup.py

@@ -95,6 +95,19 @@ class AssetValuationBackup(BaseModel):
     as_of_date: date_
 
 
+class CryptoHoldingBackup(BaseModel):
+    model_config = ConfigDict(from_attributes=True)
+
+    # asset_id doubles as this row's own primary key (see
+    # models/crypto.py's CryptoHolding) — there's no separate `id`.
+    asset_id: int
+    coingecko_id: str
+    symbol: str
+    name: str
+    thumb_url: str | None
+    quantity: Decimal
+
+
 class BudgetBackup(BaseModel):
     model_config = ConfigDict(from_attributes=True)
 
@@ -173,6 +186,9 @@ class BackupPayload(BaseModel):
     transactions: list[TransactionBackup]
     assets: list[AssetBackup]
     asset_valuations: list[AssetValuationBackup]
+    # Defaulted so a backup exported before crypto holdings existed still
+    # imports cleanly under the same format version.
+    crypto_holdings: list[CryptoHoldingBackup] = Field(default_factory=list)
     # Defaulted so a backup exported before budgets existed still imports
     # cleanly under the same format version.
     budgets: list[BudgetBackup] = Field(default_factory=list)