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/TemplateEntity.cs

@@ -0,0 +1,21 @@
+namespace HrynCo.NotificationService.DAL.EF.Entities;
+
+internal class TemplateEntity
+{
+    public Guid Id { get; set; }
+    public required string ServiceName { get; set; }
+    public required string Key { get; set; }
+    public required string LanguageCode { get; set; }
+    public required string Subject { get; set; }
+    public required string HtmlBody { get; set; }
+    public required string TextBody { get; set; }
+    public List<TemplateVariableData> Variables { get; set; } = [];
+    public DateTimeOffset Created { get; set; }
+    public DateTimeOffset? Updated { get; set; }
+}
+
+internal class TemplateVariableData
+{
+    public required string Name { get; set; }
+    public bool Required { get; set; }
+}