Feat(holidays): Implement database-backed holiday detection system

- Adds a new `MercadosFeriados` table to the database to persist market holidays.
- Implements `HolidayDataFetcher` to update holidays weekly from Finnhub API.
- Implements `IHolidayService` with in-memory caching to check for holidays efficiently.
- Worker service now skips fetcher execution on market holidays.
- Adds a new API endpoint `/api/mercados/es-feriado/{mercado}`.
- Integrates a non-blocking holiday alert into the `BolsaLocalWidget`."

frontend/src/components/MervalHeroCard.tsx

@@ -52,8 +52,8 @@ export const MervalHeroCard = () => {
         <Paper elevation={3} sx={{ p: 2, mb: 3 }}>
             <Box sx={{ display: 'flex', justifyContent: 'space-between', alignItems: 'flex-start', mb: 2 }}>
                 <Box>
-                    <Typography variant="h5" component="h2" sx={{ fontWeight: 'bold' }}>Índice S&P MERVAL</Typography>
-                    <Typography variant="h3" component="p" sx={{ fontWeight: 'bold', mt:1 }}>{formatCurrency2Decimal(mervalData.precioActual)}</Typography>
+                    <Typography variant="h6" component="h2" sx={{ fontWeight: 'bold' }}>Índice S&P MERVAL</Typography>
+                    <Typography variant="h4" component="p" sx={{ fontWeight: 'bold', mt:1 }}>{formatCurrency2Decimal(mervalData.precioActual)}</Typography>
                 </Box>
                 <Box sx={{ pt: 2 }}>
                     <VariacionMerval actual={mervalData.precioActual} anterior={mervalData.cierreAnterior} />