Convert all currencies to CRC and poll rates every 6h

Budget/transactions/salarios totals summed Transaction.amount directly,
so USD/EUR entries were treated as CRC and effectively disappeared from
the dashboard (the analytics fix in 9a80f2a only covered analytics).
Adds a shared get_converted_amount_expr() helper driven by the full
Currency enum — USD/EUR via ExchangeRate-API, BTC/XMR via CoinGecko —
and wires it into every func.sum(Transaction.amount) site.

Also starts a background task in the FastAPI lifespan that force-refreshes
every currency 4x/day, persisting USD to the DB and updating in-memory
caches for the rest. Failures are swallowed per-currency so a CoinGecko
outage cannot take out USD/EUR, and the last-known rate is always retained.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

backend/app/api/v1/endpoints/salarios.py

@@ -8,6 +8,7 @@ from sqlmodel import Session, col, func, select
 from app.auth import get_current_user
 from app.db import get_session
 from app.models.models import Transaction, TransactionRead, TransactionType
+from app.services.exchange_rate import get_converted_amount_expr
 
 router = APIRouter(prefix="/salarios", tags=["salarios"])
 
@@ -40,10 +41,11 @@ def salarios_summary(
     session: Session = Depends(get_session),
     _user: str = Depends(get_current_user),
 ):
+    amount_crc = get_converted_amount_expr(session)
     result = session.exec(
         select(
             func.count(),
-            func.coalesce(func.sum(Transaction.amount), 0),
+            func.coalesce(func.sum(amount_crc), 0),
             func.max(Transaction.date),
         ).where(Transaction.transaction_type == TransactionType.DEPOSITO)
     ).first()