Invalidate projectCode-projectId cache on deletion

Otherwise when you delete a project and create a new one with the same
code, it will return errors from GraphQL queries until the project code
to project ID cache expires, because it will still be trying to use the
GUID of the old project that was deleted.

backend/LexBoxApi/GraphQL/ProjectMutations.cs

@@ -406,6 +406,7 @@ public async Task<IQueryable<Project>> SoftDeleteProject(
         await dbContext.SaveChangesAsync();
         await hgService.SoftDeleteRepo(projectCode, timestamp);
         projectService.InvalidateProjectConfidentialityCache(projectId);
+        projectService.InvalidateProjectCodeCache(projectCode);
         await transaction.CommitAsync();
 
         return dbContext.Projects.Where(p => p.Id == projectId);

backend/LexBoxApi/Services/ProjectService.cs

@@ -96,6 +96,12 @@ public async ValueTask<Guid> LookupProjectId(string projectCode)
         return projectId;
     }
 
+    public void InvalidateProjectCodeCache(string projectCode)
+    {
+        try { memoryCache.Remove($"ProjectIdForCode:{projectCode}"); }
+        catch (Exception) { }; // Never allow this to throw
+    }
+
     public async Task<BackupExecutor?> BackupProject(string code)
     {
         var exists = await dbContext.Projects.Where(p => p.Code == code)