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/analytics.py

@@ -10,7 +10,7 @@ from app.auth import get_current_user
 from app.db import get_session
 from app.models.models import Category, Transaction
 from app.services.budget_projection import get_cycle_range
-from app.services.exchange_rate import get_current_rate, get_eur_crc_rate
+from app.services.exchange_rate import get_converted_amount_expr
 
 router = APIRouter(prefix="/analytics", tags=["analytics"])
 
@@ -38,17 +38,6 @@ class DailySpending(BaseModel):
     count: int
 
 
-def _get_crc_multipliers(session: Session) -> dict[str, float]:
-    """Return multipliers to convert each currency to CRC."""
-    usd_rate = get_current_rate(session)
-    eur_rate = get_eur_crc_rate()
-    return {
-        "CRC": 1.0,
-        "USD": usd_rate.sell_rate if usd_rate else 0.0,
-        "EUR": eur_rate if eur_rate else 0.0,
-    }
-
-
 @router.get("/by-category", response_model=list[CategorySpending])
 def spending_by_category(
     cycle_year: Optional[int] = None,
@@ -56,18 +45,12 @@ def spending_by_category(
     session: Session = Depends(get_session),
     _user: str = Depends(get_current_user),
 ):
-    rates = _get_crc_multipliers(session)
+    amount_crc = get_converted_amount_expr(session)
 
     query = (
         select(
             Transaction.category_id,
-            func.sum(
-                case(
-                    (Transaction.currency == "USD", Transaction.amount * rates["USD"]),
-                    (Transaction.currency == "EUR", Transaction.amount * rates["EUR"]),
-                    else_=Transaction.amount,
-                )
-            ).label("total"),
+            func.sum(amount_crc).label("total"),
             func.count().label("count"),
         )
         .where(Transaction.transaction_type == "COMPRA")
@@ -113,7 +96,7 @@ def monthly_trend(
     total_crc includes all currencies converted to CRC at current rates.
     total_usd is the raw USD amount (unconverted) for display purposes.
     """
-    rates = _get_crc_multipliers(session)
+    amount_crc = get_converted_amount_expr(session)
     now = datetime.now()
     results = []
     month_names = [
@@ -128,16 +111,7 @@ def monthly_trend(
         row = session.exec(
             select(
                 func.count(),
-                func.coalesce(
-                    func.sum(
-                        case(
-                            (Transaction.currency == "USD", Transaction.amount * rates["USD"]),
-                            (Transaction.currency == "EUR", Transaction.amount * rates["EUR"]),
-                            else_=Transaction.amount,
-                        )
-                    ),
-                    0,
-                ),
+                func.coalesce(func.sum(amount_crc), 0),
                 func.coalesce(
                     func.sum(
                         case(
@@ -189,18 +163,12 @@ def daily_spending(
     session: Session = Depends(get_session),
     _user: str = Depends(get_current_user),
 ):
-    rates = _get_crc_multipliers(session)
+    amount_crc = get_converted_amount_expr(session)
 
     query = (
         select(
             func.date(Transaction.date).label("day"),
-            func.sum(
-                case(
-                    (Transaction.currency == "USD", Transaction.amount * rates["USD"]),
-                    (Transaction.currency == "EUR", Transaction.amount * rates["EUR"]),
-                    else_=Transaction.amount,
-                )
-            ).label("total"),
+            func.sum(amount_crc).label("total"),
             func.count().label("count"),
         )
         .where(Transaction.transaction_type == "COMPRA")