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/NotificationDbContext.cs

@@ -0,0 +1,21 @@
+using HrynCo.NotificationService.DAL.EF.Entities;
+using Microsoft.EntityFrameworkCore;
+
+namespace HrynCo.NotificationService.DAL.EF;
+
+public class NotificationDbContext : DbContext
+{
+    public NotificationDbContext(DbContextOptions<NotificationDbContext> options)
+        : base(options)
+    {
+    }
+
+    internal DbSet<TemplateEntity> Templates => Set<TemplateEntity>();
+    internal DbSet<ProviderEntity> Providers => Set<ProviderEntity>();
+    internal DbSet<ProviderUsageEntity> ProviderUsage => Set<ProviderUsageEntity>();
+
+    protected override void OnModelCreating(ModelBuilder modelBuilder)
+    {
+        modelBuilder.ApplyConfigurationsFromAssembly(typeof(NotificationDbContext).Assembly);
+    }
+}