Add winner archive, host image upload and live demo data

Deliver the demo-ready feature set and seed data so the live site can be
presented end to end:

- Winner archive: ArchivedWinner domain, admin CRUD endpoints/view/manager
  and public archive surface, backed by AddArchivedWinners migration.
- Host presentation: host image upload and artist name on SiteSettings with
  public image endpoint and supporting migrations.
- Clip submissions: idempotent table-ensure migration plus current-season
  demo clips for review workflows.
- Demo seed data: sponsors, share links and 2025 archived winners, with a
  guarded RemoveDemoSeasons cleanup; all seeds guard against real data.
- EnsureRuntimeSchemaParity migration to align runtime schema defensively.
- Admin/home UI refinements; remove unused team role permissions modal and
  dead share-quick-links code.

All seed and schema migrations are idempotent (IF NOT EXISTS / ON CONFLICT)
and skip when real season data is present.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

Backend/Migrations/20260629181644_AddArchivedWinners.cs

@@ -0,0 +1,48 @@
+using System;
+using Microsoft.EntityFrameworkCore.Migrations;
+using Npgsql.EntityFrameworkCore.PostgreSQL.Metadata;
+
+#nullable disable
+
+namespace Backend.Migrations
+{
+    /// <inheritdoc />
+    public partial class AddArchivedWinners : Migration
+    {
+        /// <inheritdoc />
+        protected override void Up(MigrationBuilder migrationBuilder)
+        {
+            migrationBuilder.CreateTable(
+                name: "ArchivedWinners",
+                columns: table => new
+                {
+                    Id = table.Column<int>(type: "integer", nullable: false)
+                        .Annotation("Npgsql:ValueGenerationStrategy", NpgsqlValueGenerationStrategy.IdentityByDefaultColumn),
+                    Year = table.Column<int>(type: "integer", nullable: false),
+                    Category = table.Column<string>(type: "character varying(120)", maxLength: 120, nullable: false),
+                    Subcategory = table.Column<string>(type: "character varying(120)", maxLength: 120, nullable: false),
+                    WinnerName = table.Column<string>(type: "character varying(120)", maxLength: 120, nullable: false),
+                    WinnerUrl = table.Column<string>(type: "character varying(500)", maxLength: 500, nullable: false),
+                    CreatedAt = table.Column<DateTimeOffset>(type: "timestamp with time zone", nullable: false),
+                    UpdatedAt = table.Column<DateTimeOffset>(type: "timestamp with time zone", nullable: true)
+                },
+                constraints: table =>
+                {
+                    table.PrimaryKey("PK_ArchivedWinners", x => x.Id);
+                });
+
+            migrationBuilder.CreateIndex(
+                name: "IX_ArchivedWinners_Year_Category_Subcategory",
+                table: "ArchivedWinners",
+                columns: new[] { "Year", "Category", "Subcategory" },
+                unique: true);
+        }
+
+        /// <inheritdoc />
+        protected override void Down(MigrationBuilder migrationBuilder)
+        {
+            migrationBuilder.DropTable(
+                name: "ArchivedWinners");
+        }
+    }
+}