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/20260629194000_RemoveDemoSeasons.cs

@@ -0,0 +1,33 @@
+using Microsoft.EntityFrameworkCore.Infrastructure;
+using Microsoft.EntityFrameworkCore.Migrations;
+
+#nullable disable
+
+namespace Backend.Migrations;
+
+[DbContext(typeof(Data.AwardsDbContext))]
+[Migration("20260629194000_RemoveDemoSeasons")]
+public partial class RemoveDemoSeasons : Migration
+{
+    protected override void Up(MigrationBuilder migrationBuilder)
+    {
+        migrationBuilder.Sql(
+            """
+            UPDATE "RiskFlags"
+            SET "SeasonId" = NULL
+            WHERE "SeasonId" IN (
+                SELECT "Id"
+                FROM "Seasons"
+                WHERE "IsDemo" = TRUE
+            );
+
+            DELETE FROM "Seasons"
+            WHERE "IsDemo" = TRUE;
+            """
+        );
+    }
+
+    protected override void Down(MigrationBuilder migrationBuilder)
+    {
+    }
+}