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/Common/SeasonMappings.cs

@@ -92,6 +92,50 @@ public static class SeasonMappings
         };
     }
 
+    public static (string Platform, string Slug) InferProfileMetadataFromUrl(string? value, string fallbackName = "")
+    {
+        var trimmed = value?.Trim() ?? string.Empty;
+        if (!Uri.TryCreate(trimmed, UriKind.Absolute, out var uri))
+        {
+            return ("Profil", fallbackName.Trim());
+        }
+
+        var host = uri.Host.Trim().ToLowerInvariant();
+        var segments = uri.AbsolutePath
+            .Split('/', StringSplitOptions.RemoveEmptyEntries | StringSplitOptions.TrimEntries);
+
+        var platform = host switch
+        {
+            var item when item.Contains("twitch.tv", StringComparison.Ordinal) => "Twitch",
+            var item when item.Contains("youtube.com", StringComparison.Ordinal) || item.Contains("youtu.be", StringComparison.Ordinal) => "YouTube",
+            var item when item.Contains("x.com", StringComparison.Ordinal) || item.Contains("twitter.com", StringComparison.Ordinal) => "X",
+            var item when item.Contains("instagram.com", StringComparison.Ordinal) => "Instagram",
+            var item when item.Contains("discord.gg", StringComparison.Ordinal) || item.Contains("discord.com", StringComparison.Ordinal) => "Discord",
+            var item when item.Contains("kick.com", StringComparison.Ordinal) => "Kick",
+            var item when item.Contains("cake.gg", StringComparison.Ordinal) => "Cake",
+            _ => "Profil",
+        };
+
+        var slug = segments.LastOrDefault() ?? string.Empty;
+        if (string.Equals(platform, "YouTube", StringComparison.Ordinal) && segments.Length > 0)
+        {
+            slug = segments.FirstOrDefault(segment => segment.StartsWith('@')) ?? slug;
+        }
+
+        slug = Uri.UnescapeDataString(slug).Trim().Trim('/');
+        if (slug.StartsWith('@') && !string.Equals(platform, "YouTube", StringComparison.Ordinal))
+        {
+            slug = slug[1..];
+        }
+
+        if (string.IsNullOrWhiteSpace(slug))
+        {
+            slug = fallbackName.Trim();
+        }
+
+        return (platform, slug);
+    }
+
     public static string NormalizeSeasonStreamUrl(string? value)
     {
         var trimmed = value?.Trim() ?? string.Empty;