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/Contracts/AdminArchiveContracts.cs

@@ -0,0 +1,18 @@
+namespace Backend.Contracts;
+
+public sealed record AdminArchivedWinnerItemDto(
+    int Id,
+    int Year,
+    string Category,
+    string Subcategory,
+    string WinnerName,
+    string WinnerUrl,
+    DateTimeOffset CreatedAt,
+    DateTimeOffset? UpdatedAt);
+
+public sealed record UpsertArchivedWinnerRequest(
+    int Year,
+    string Category,
+    string Subcategory,
+    string WinnerName,
+    string WinnerUrl);

Backend/Contracts/AdminSeasonContracts.cs

@@ -37,6 +37,8 @@ public sealed record AdminCandidateItemDto(
     string DisplayName,
     string ChannelSlug,
     string Platform,
+    int? AvgViewers,
+    int Votes,
     int NominationTally,
     string AcceptanceStatus,
     string? AcceptanceNote,

Backend/Contracts/AdminSiteSettingsContracts.cs

@@ -3,6 +3,8 @@ namespace Backend.Contracts;
 public sealed record AdminSiteSettingsResponse(
     string HostDisplayName,
     string HostTagline,
+    string HostArtistName,
+    string HostImageUrl,
     string NewsletterUrl,
     string ShareXUrl,
     string ShareDiscordUrl,
@@ -41,6 +43,7 @@ public sealed record AdminSiteSettingsResponse(
 public sealed record UpdateSiteSettingsRequest(
     string HostDisplayName,
     string HostTagline,
+    string HostArtistName,
     string NewsletterUrl,
     string ShareXUrl,
     string ShareDiscordUrl,

Backend/Contracts/PublicOverviewContracts.cs

@@ -64,6 +64,8 @@ public sealed record PublicStreamBannerContentDto(
 public sealed record PublicSiteContentDto(
     string HostDisplayName,
     string HostTagline,
+    string HostArtistName,
+    string HostImageUrl,
     string NewsletterUrl,
     string ShareXUrl,
     string ShareDiscordUrl,