Adds the Offerings module and retargets the CI/CD pipeline to Api.SlpSoftware

Implements Unit 2 "Offerings" (backend module, admin CRUD UI with
drag-and-drop reordering, public GET /api/v1/offerings endpoint) and
executes the feature's D-15 CI/CD cutover, switching the deploy
pipeline's build/publish target from SlpModularCms.Api to
SlpModularCms.Api.SlpSoftware.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01FWyStNL2ZsjrS7FLd7xvvN

src/SlpModularCms.Modules.Offerings/Models/CreateOfferingRequest.cs

@@ -0,0 +1,13 @@
+using System.ComponentModel.DataAnnotations;
+
+namespace SlpModularCms.Modules.Offerings.Models;
+
+public record CreateOfferingRequest(
+    [Required, MaxLength(100)] string Title,
+    [Required, MaxLength(500)] string Description,
+    [Required, MaxLength(50)] string Price,
+    [Required, MaxLength(100)] string PriceNote,
+    [Required, FeaturesValidation] List<string> Features,
+    [Required, MaxLength(50)] string CtaLabel,
+    bool Featured = false
+);