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/Domain/ArchivedWinner.cs

@@ -0,0 +1,13 @@
+namespace Backend.Domain;
+
+public sealed class ArchivedWinner
+{
+    public int Id { get; set; }
+    public int Year { get; set; }
+    public string Category { get; set; } = string.Empty;
+    public string Subcategory { get; set; } = string.Empty;
+    public string WinnerName { get; set; } = string.Empty;
+    public string WinnerUrl { get; set; } = string.Empty;
+    public DateTimeOffset CreatedAt { get; set; } = DateTimeOffset.UtcNow;
+    public DateTimeOffset? UpdatedAt { get; set; }
+}

Backend/Domain/SiteSettings.cs

@@ -5,6 +5,10 @@ public sealed class SiteSettings
     public int Id { get; set; }
     public string HostDisplayName { get; set; } = string.Empty;
     public string HostTagline { get; set; } = string.Empty;
+    public string HostArtistName { get; set; } = string.Empty;
+    public byte[]? HostImageData { get; set; }
+    public string? HostImageContentType { get; set; }
+    public DateTimeOffset? HostImageUpdatedAt { get; set; }
     public string NewsletterUrl { get; set; } = string.Empty;
     public string ShareXUrl { get; set; } = string.Empty;
     public string ShareDiscordUrl { get; set; } = string.Empty;