feat(api): state filter + populated num_issues on project columns

Adds two improvements to the user/org/repo project-board REST API:

* state filter on column-issues endpoints (issue #4)
  GET /api/v1/{users,orgs}/{name}/-/projects/{id}/columns/{col}/issues
  GET /api/v1/repos/{owner}/{repo}/projects/{id}/columns/{col}/issues
  Now accept ?state=open|closed|all (default open), matching the convention
  on the project-list endpoint and on /repos/.../issues. Applied at the
  IssuesOptions layer so all three scopes inherit the filter.

* populated num_issues / num_open_issues / num_closed_issues on column-list
  (issue #5)
  ColumnList.LoadIssueCounts runs two grouped queries against project_issue
  joined with issue (one open, one closed). All three List*Columns handlers
  call it before converting, so num_issues stops being null and consumers
  can render a kanban summary in a single round trip instead of N+1.

Tests:
* unit: empty-input fast path on ColumnList.LoadIssueCounts.
* integration: extended testAPIListProjectColumnIssues / -User / -Org to
  close an issue, then verify default=open hides it, state=closed and
  state=all return it, and the column-list response carries the correct
  open/closed/total split.

Closes #4, closes #5

services/convert/project.go

@@ -147,15 +147,17 @@ func ToProjectColumn(ctx context.Context, column *project_model.Column, doer *us
 
 func toProjectColumn(ctx context.Context, column *project_model.Column, doer *user_model.User, creators map[int64]*user_model.User) *api.ProjectColumn {
 	apiColumn := &api.ProjectColumn{
-		ID:        column.ID,
-		Title:     column.Title,
-		Default:   column.Default,
-		Sorting:   int(column.Sorting),
-		Color:     column.Color,
-		ProjectID: column.ProjectID,
-		NumIssues: column.NumIssues,
-		CreatedAt: column.CreatedUnix.AsTime(),
-		UpdatedAt: column.UpdatedUnix.AsTime(),
+		ID:              column.ID,
+		Title:           column.Title,
+		Default:         column.Default,
+		Sorting:         int(column.Sorting),
+		Color:           column.Color,
+		ProjectID:       column.ProjectID,
+		NumIssues:       column.NumIssues,
+		NumOpenIssues:   column.NumOpenIssues,
+		NumClosedIssues: column.NumClosedIssues,
+		CreatedAt:       column.CreatedUnix.AsTime(),
+		UpdatedAt:       column.UpdatedUnix.AsTime(),
 	}
 	if creator, ok := creators[column.CreatorID]; ok {
 		apiColumn.Creator = ToUser(ctx, creator, doer)