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/AdminSeasonCandidateEndpoints.cs

@@ -193,6 +193,34 @@ public static partial class AdminSeasonManagementEndpoints
         return Results.Ok(new { saved = true, candidateId = candidate.Id });
     }
 
+    private static async Task<IResult> GetCandidateDeletePreview(
+        int candidateId,
+        AwardsDbContext db,
+        CancellationToken cancellationToken)
+    {
+        var candidate = await db.Candidates
+            .AsNoTracking()
+            .FirstOrDefaultAsync(item => item.Id == candidateId, cancellationToken);
+        if (candidate is null)
+        {
+            return Results.NotFound();
+        }
+
+        var nominationCount = await db.Nominations.CountAsync(item => item.CandidateId == candidateId, cancellationToken);
+        var clipCount = await db.ClipSubmissions.CountAsync(item => item.CandidateId == candidateId, cancellationToken);
+        var voteCount = await db.VoteEntries.CountAsync(item => item.CandidateId == candidateId, cancellationToken);
+        var resultCount = await db.Results.CountAsync(item => item.CandidateId == candidateId, cancellationToken);
+
+        return Results.Ok(new
+        {
+            candidateId,
+            nominationCount,
+            clipCount,
+            voteCount,
+            resultCount,
+        });
+    }
+
     private static async Task<IResult> DeleteCandidate(
         HttpContext context,
         int candidateId,
@@ -206,6 +234,38 @@ public static partial class AdminSeasonManagementEndpoints
             return Results.NotFound();
         }
 
+        var linkedNominations = await db.Nominations
+            .Where(item => item.CandidateId == candidateId)
+            .ToListAsync(context.RequestAborted);
+        if (linkedNominations.Count > 0)
+        {
+            db.Nominations.RemoveRange(linkedNominations);
+        }
+
+        var linkedClips = await db.ClipSubmissions
+            .Where(item => item.CandidateId == candidateId)
+            .ToListAsync(context.RequestAborted);
+        if (linkedClips.Count > 0)
+        {
+            db.ClipSubmissions.RemoveRange(linkedClips);
+        }
+
+        var linkedVoteEntries = await db.VoteEntries
+            .Where(item => item.CandidateId == candidateId)
+            .ToListAsync(context.RequestAborted);
+        if (linkedVoteEntries.Count > 0)
+        {
+            db.VoteEntries.RemoveRange(linkedVoteEntries);
+        }
+
+        var linkedResults = await db.Results
+            .Where(item => item.CandidateId == candidateId)
+            .ToListAsync(context.RequestAborted);
+        if (linkedResults.Count > 0)
+        {
+            db.Results.RemoveRange(linkedResults);
+        }
+
         db.Candidates.Remove(candidate);
         adminAuditService.AddEntry(
             session.TwitchUserId,
@@ -213,11 +273,29 @@ public static partial class AdminSeasonManagementEndpoints
             "candidate",
             candidate.Id.ToString(),
             $"Kandidat {candidate.DisplayName} wurde gelöscht.",
-            new { candidate.CategoryId, candidate.Platform },
+            new
+            {
+                candidate.CategoryId,
+                candidate.Platform,
+                deletedNominations = linkedNominations.Count,
+                deletedClips = linkedClips.Count,
+                deletedVoteEntries = linkedVoteEntries.Count,
+                deletedResults = linkedResults.Count,
+            },
             RequestMetadataReader.Read(context));
 
-        await db.SaveChangesAsync(context.RequestAborted);
-        return Results.Ok(new { deleted = true, candidateId });
+        try
+        {
+            await db.SaveChangesAsync(context.RequestAborted);
+            return Results.Ok(new { deleted = true, candidateId });
+        }
+        catch (DbUpdateException)
+        {
+            return Results.BadRequest(new
+            {
+                message = "Kandidat konnte nicht gelöscht werden, weil noch verknüpfte Daten blockieren.",
+            });
+        }
     }
 
     private static string NormalizeCandidateChoice(string? value, string fallback, IReadOnlyCollection<string> allowedValues)