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/Endpoints/AdminSeasonManagementEndpoints.cs

@@ -60,6 +60,10 @@ public static partial class AdminSeasonManagementEndpoints
             .AddEndpointFilter((context, next) => AdminEndpointConventions.RequireWritePermission(context, next, Backend.Security.AdminPermissionCatalog.Candidates))
             .WithName("UpdateAdminCandidate")
             .WithOpenApi();
+        group.MapGet("/candidates/{candidateId:int}/delete-preview", GetCandidateDeletePreview)
+            .AddEndpointFilter((context, next) => AdminEndpointConventions.RequireWritePermission(context, next, Backend.Security.AdminPermissionCatalog.Candidates))
+            .WithName("GetAdminCandidateDeletePreview")
+            .WithOpenApi();
         group.MapDelete("/candidates/{candidateId:int}", DeleteCandidate)
             .AddEndpointFilter((context, next) => AdminEndpointConventions.RequireWritePermission(context, next, Backend.Security.AdminPermissionCatalog.Candidates))
             .WithName("DeleteAdminCandidate")