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/transactions.py

@@ -20,6 +20,7 @@ from app.models.models import (
 )
 
 from app.services.budget_projection import get_cycle_range, get_previous_cycle
+from app.services.exchange_rate import get_converted_amount_expr
 
 router = APIRouter(prefix="/transactions", tags=["transactions"])
 
@@ -110,6 +111,7 @@ def list_billing_cycles(
         return []
 
     min_date, max_date = result
+    amount_crc = get_converted_amount_expr(session)
 
     cycles = []
     # Determine which cycle the min_date falls into
@@ -129,7 +131,7 @@ def list_billing_cycles(
 
         # Count transactions in this cycle
         count_result = session.exec(
-            select(func.count(), func.coalesce(func.sum(Transaction.amount), 0)).where(
+            select(func.count(), func.coalesce(func.sum(amount_crc), 0)).where(
                 Transaction.date >= start, Transaction.date < end
             )
         ).first()