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/Entities/ProviderEntity.cs

@@ -0,0 +1,24 @@
+using HrynCo.NotificationService.DAL.Abstract.Providers;
+
+namespace HrynCo.NotificationService.DAL.EF.Entities;
+
+internal class ProviderEntity
+{
+    public Guid Id { get; set; }
+    public required string ServiceName { get; set; }
+    public int Priority { get; set; }
+    public ProviderType ProviderType { get; set; }
+
+    /// <summary>
+    /// Provider-specific credentials and settings stored as JSONB.
+    /// Deserialized based on <see cref="ProviderType"/> in the repository.
+    /// </summary>
+    public required string SettingsJson { get; set; }
+
+    public int? DailyLimit { get; set; }
+    public int? MonthlyLimit { get; set; }
+    public int WarnThresholdPercent { get; set; }
+    public bool IsActive { get; set; }
+    public DateTimeOffset Created { get; set; }
+    public DateTimeOffset? Updated { get; set; }
+}