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/main.py

@@ -11,6 +11,7 @@ from app.api.routes import (
     budgets,
     cash_flow,
     categories,
+    crypto,
     dashboard,
     goals,
     insights,
@@ -63,6 +64,7 @@ app.include_router(goals.router, prefix="/api")
 app.include_router(recurring.router, prefix="/api")
 app.include_router(cash_flow.router, prefix="/api")
 app.include_router(tags.router, prefix="/api")
+app.include_router(crypto.router, prefix="/api")
 
 
 @app.get("/api/health")