Blog: enforce Restrict FK on BlogPost.Author and Comment.Author

Tout billet a un auteur, tout commentaire a un auteur. On aligne la
base sur ce contrat (Postgres) en droppant les orphelins existants
puis en remplaçant les FK en cascade par des FK Restrict.

- ApplicationDbContext: fluent pour BlogPost.Author et Comment.Author
  en DeleteBehavior.Restrict.
- ApplicationUser: ajoute la nav inverse BlogComments (manquait,
  EF aurait sinon créé une shadow FK).
- Migration 20260711173717_EnforceBlogAuthorFKs: Up purge les
  Comment/BlogSpot dont l'AuthorId n'existe plus, log le volume,
  puis drop+add des FK. Down laisse la cascade (état pré-migration).

Le code applicatif (BlogSpotService.Details) s'appuiera sur cette
contrainte dans un commit séparé.

src/Yavsc.Org/Migrations/20260711173717_EnforceBlogAuthorFKs.cs

@@ -0,0 +1,89 @@
+using Microsoft.EntityFrameworkCore.Migrations;
+
+#nullable disable
+
+namespace Yavsc.Migrations
+{
+    /// <inheritdoc />
+    public partial class EnforceBlogAuthorFKs : Migration
+    {
+        /// <inheritdoc />
+        protected override void Up(MigrationBuilder migrationBuilder)
+        {
+            // Assainir les orphelins AVANT d'enforcer la FK Restrict.
+            // En prod (Postgres), la migration aurait sinon planté
+            // sur des billets/commentaires dont l'AuthorId pointe
+            // vers un user déjà supprimé. La logique métier refuse
+            // désormais l'orphelin (cf. BlogSpotService.Details) — on
+            // aligne l'état de la base avec ce contrat.
+            migrationBuilder.Sql(@"
+                DO $$
+                DECLARE n_comments int;
+                        n_posts    int;
+                BEGIN
+                    DELETE FROM ""Comment""
+                    WHERE ""AuthorId"" NOT IN (SELECT ""Id"" FROM ""AspNetUsers"");
+                    GET DIAGNOSTICS n_comments = ROW_COUNT;
+
+                    DELETE FROM ""BlogSpot""
+                    WHERE ""AuthorId"" NOT IN (SELECT ""Id"" FROM ""AspNetUsers"");
+                    GET DIAGNOSTICS n_posts = ROW_COUNT;
+
+                    RAISE NOTICE 'EnforceBlogAuthorFKs: % orphaned comments deleted, % orphaned blog posts deleted',
+                        n_comments, n_posts;
+                END $$;
+            ");
+
+            migrationBuilder.DropForeignKey(
+                name: "FK_BlogSpot_AspNetUsers_AuthorId",
+                table: "BlogSpot");
+
+            migrationBuilder.DropForeignKey(
+                name: "FK_Comment_AspNetUsers_AuthorId",
+                table: "Comment");
+
+            migrationBuilder.AddForeignKey(
+                name: "FK_BlogSpot_AspNetUsers_AuthorId",
+                table: "BlogSpot",
+                column: "AuthorId",
+                principalTable: "AspNetUsers",
+                principalColumn: "Id",
+                onDelete: ReferentialAction.Restrict);
+
+            migrationBuilder.AddForeignKey(
+                name: "FK_Comment_AspNetUsers_AuthorId",
+                table: "Comment",
+                column: "AuthorId",
+                principalTable: "AspNetUsers",
+                principalColumn: "Id",
+                onDelete: ReferentialAction.Restrict);
+        }
+
+        /// <inheritdoc />
+        protected override void Down(MigrationBuilder migrationBuilder)
+        {
+            migrationBuilder.DropForeignKey(
+                name: "FK_BlogSpot_AspNetUsers_AuthorId",
+                table: "BlogSpot");
+
+            migrationBuilder.DropForeignKey(
+                name: "FK_Comment_AspNetUsers_AuthorId",
+                table: "Comment");
+
+            migrationBuilder.AddForeignKey(
+                name: "FK_BlogSpot_AspNetUsers_AuthorId",
+                table: "BlogSpot",
+                column: "AuthorId",
+                principalTable: "AspNetUsers",
+                principalColumn: "Id");
+
+            migrationBuilder.AddForeignKey(
+                name: "FK_Comment_AspNetUsers_AuthorId",
+                table: "Comment",
+                column: "AuthorId",
+                principalTable: "AspNetUsers",
+                principalColumn: "Id",
+                onDelete: ReferentialAction.Cascade);
+        }
+    }
+}

src/Yavsc.Org/Migrations/ApplicationDbContextModelSnapshot.cs

@@ -3803,7 +3803,8 @@ namespace Yavsc.Migrations
                 {
                     b.HasOne("Yavsc.Models.ApplicationUser", "Author")
                         .WithMany("Posts")
-                        .HasForeignKey("AuthorId");
+                        .HasForeignKey("AuthorId")
+                        .OnDelete(DeleteBehavior.Restrict);
 
                     b.Navigation("Author");
                 });
@@ -3830,9 +3831,9 @@ namespace Yavsc.Migrations
             modelBuilder.Entity("Yavsc.Models.Blog.Comment", b =>
                 {
                     b.HasOne("Yavsc.Models.ApplicationUser", "Author")
-                        .WithMany()
+                        .WithMany("BlogComments")
                         .HasForeignKey("AuthorId")
-                        .OnDelete(DeleteBehavior.Cascade)
+                        .OnDelete(DeleteBehavior.Restrict)
                         .IsRequired();
 
                     b.HasOne("Yavsc.Models.Blog.Comment", "Parent")
@@ -4542,6 +4543,8 @@ namespace Yavsc.Migrations
 
                     b.Navigation("BlackList");
 
+                    b.Navigation("BlogComments");
+
                     b.Navigation("Book");
 
                     b.Navigation("Circles");