fix(api): validate block group exists on block creation

POST /api/blocks with a nonexistent blockGroupId returned an
unhandled DbUpdateException (HTTP 500) because CreateBlockHandler
never checked the referenced block group before inserting, letting
the SQLite FK constraint raise. Add a BaseError validation (422),
matching the "does not exist" precedent already used elsewhere.

Found via live E2E testing.

ErsatzTV.Application/Scheduling/Commands/CreateBlockHandler.cs

@@ -25,13 +25,29 @@ public class CreateBlockHandler(IDbContextFactory<TvContext> dbContextFactory)
         return Mapper.ProjectToViewModel(block);
     }
 
-    private static async Task<Validation<BaseError, Block>> Validate(TvContext dbContext, CreateBlock request) =>
-        await ValidateBlockName(dbContext, request).MapT(name => new Block
+    private static async Task<Validation<BaseError, Block>> Validate(TvContext dbContext, CreateBlock request)
+    {
+        Validation<BaseError, Unit> blockGroupValidation = await ValidateBlockGroupExists(dbContext, request);
+        Validation<BaseError, string> nameValidation = await ValidateBlockName(dbContext, request);
+
+        return (blockGroupValidation, nameValidation).Apply((_, name) => new Block
         {
             BlockGroupId = request.BlockGroupId,
             Name = name,
             Minutes = 30
         });
+    }
+
+    private static async Task<Validation<BaseError, Unit>> ValidateBlockGroupExists(
+        TvContext dbContext,
+        CreateBlock request)
+    {
+        bool blockGroupExists = await dbContext.BlockGroups.AnyAsync(bg => bg.Id == request.BlockGroupId);
+
+        return blockGroupExists
+            ? Success<BaseError, Unit>(Unit.Default)
+            : BaseError.New("Block group does not exist");
+    }
 
     private static async Task<Validation<BaseError, string>> ValidateBlockName(
         TvContext dbContext,