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/crypto.py

@@ -0,0 +1,60 @@
+from datetime import datetime
+from decimal import Decimal
+from typing import Literal
+
+from pydantic import BaseModel, ConfigDict, Field
+
+
+class CryptoHoldingCreate(BaseModel):
+    coingecko_id: str = Field(min_length=1, max_length=100)
+    symbol: str = Field(min_length=1, max_length=20)
+    name: str = Field(min_length=1, max_length=150)
+    thumb_url: str | None = Field(default=None, max_length=500)
+    # 18 decimal places covers wei-level token amounts; still gt=0 since a
+    # zero/negative holding isn't a holding.
+    quantity: Decimal = Field(gt=0, max_digits=38, decimal_places=18)
+
+
+class CryptoHoldingUpdate(BaseModel):
+    quantity: Decimal = Field(gt=0, max_digits=38, decimal_places=18)
+
+
+class CryptoHoldingRead(BaseModel):
+    model_config = ConfigDict(from_attributes=True)
+
+    asset_id: int
+    coingecko_id: str
+    symbol: str
+    name: str
+    thumb_url: str | None
+    quantity: Decimal
+    # Both derived from the linked Asset's latest AssetValuation — null
+    # value/unit_price means "added but never priced yet" (CoinGecko was
+    # unreachable at creation time), not zero.
+    value: Decimal | None
+    unit_price: Decimal | None
+    as_of_date: str | None
+
+
+class CryptoSyncResult(BaseModel):
+    # False when the lazy daily check ran but the 24h window hadn't
+    # elapsed yet (nothing called CoinGecko this time, error_key is None),
+    # OR when a refresh was attempted but CoinGecko couldn't be reached
+    # (error_key is set) — check error_key to tell the two apart. Either
+    # way `holdings` still carries the best data on hand, never blocked by
+    # an external outage.
+    #
+    # A key, not a message string: the frontend translates it (RU/EN), same
+    # key+params convention as FinancialAlert/AdviceItem — a raw English
+    # sentence here would silently break the app's bilingual UI.
+    synced: bool
+    last_synced_at: datetime | None
+    error_key: Literal["unreachable"] | None = None
+    holdings: list[CryptoHoldingRead]
+
+
+class CryptoSearchResult(BaseModel):
+    coingecko_id: str
+    symbol: str
+    name: str
+    thumb_url: str | None