feat: add EF Core layer with entities, configurations and DbContext

- Directory.Packages.props and Directory.Build.props for central package management
- TemplateEntity, ProviderEntity, ProviderUsageEntity (internal to DAL.EF)
- TemplateEntityConfiguration: composite unique index (service_name, key, language_code), Variables as JSON column
- ProviderEntityConfiguration: settings stored as jsonb, index on (service_name, priority)
- ProviderUsageEntityConfiguration: composite unique index (provider_id, date)
- All entities map Id column explicitly as 'id' (snake_case)
- NotificationDbContext with ApplyConfigurationsFromAssembly

Ref: IT-628

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

HrynCo.NotificationService.DAL.EF/Configurations/ProviderUsageEntityConfiguration.cs

@@ -0,0 +1,25 @@
+using HrynCo.NotificationService.DAL.EF.Entities;
+using Microsoft.EntityFrameworkCore;
+using Microsoft.EntityFrameworkCore.Metadata.Builders;
+
+namespace HrynCo.NotificationService.DAL.EF.Configurations;
+
+internal class ProviderUsageEntityConfiguration : IEntityTypeConfiguration<ProviderUsageEntity>
+{
+    public void Configure(EntityTypeBuilder<ProviderUsageEntity> builder)
+    {
+        builder.ToTable("provider_usage");
+
+        builder.HasKey(x => x.Id);
+        builder.Property(x => x.Id).HasColumnName("id");
+
+        builder.Property(x => x.ProviderId).HasColumnName("provider_id");
+
+        builder.HasIndex(x => new { x.ProviderId, x.Date }).IsUnique();
+
+        builder.Property(x => x.Date).HasColumnName("date");
+        builder.Property(x => x.SentCount).HasColumnName("sent_count");
+        builder.Property(x => x.Created).HasColumnName("created");
+        builder.Property(x => x.Updated).HasColumnName("updated");
+    }
+}