feat(tenant): bill AI points by output characters

Previously AI point cost was computed from request (input) characters at
reservation time. Switch to reserving a single point up front and
settling the final cost from the generated output length on completion.

- Reserve 1 point (or FixedPoints) instead of pricing on request chars
- On completion, recompute points from output chars vs base chars,
  update the reservation amounts, and post a ledger delta for the
  top-up/refund; invalidate the workspace quota summary cache
- MarkCompleted now persists final request_chars/base_chars/points
- Use ceil division in calculateAIPointCost so an exact base boundary
  charges one point instead of rolling over
- Thread output text through all CompleteAIPoints callers (article
  selection, KOL assist, question expansion, template assist, compliance
  judge) and return the settled reservation
- Add unit tests plus an integration test gated on TEST_DATABASE_URL
- Update the user manual: billing is by output characters

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

server/internal/worker/assist/kol_assist_worker.go

@@ -93,7 +93,7 @@ func (w *KolAssistWorker) Process(ctx context.Context, job tenantapp.KolAssistJo
 	}
 	switch task.Status {
 	case "completed":
-		w.completeAIPoints(ctx, job, "")
+		w.completeAIPoints(ctx, job, "", string(task.ResultJSON))
 		return nil
 	case "failed":
 		errorMessage := "kol assist task failed"
@@ -262,14 +262,14 @@ func (w *KolAssistWorker) processTask(ctx context.Context, job tenantapp.KolAssi
 	if err := w.repo.MarkCompleted(ctx, job.TenantID, job.TaskID, resultJSON); err != nil {
 		return fmt.Errorf("mark kol assist completed: %w", err)
 	}
-	w.completeAIPoints(ctx, job, result.Model)
+	w.completeAIPoints(ctx, job, result.Model, result.Content)
 	return nil
 }
 
-func (w *KolAssistWorker) completeAIPoints(ctx context.Context, job tenantapp.KolAssistJob, model string) {
+func (w *KolAssistWorker) completeAIPoints(ctx context.Context, job tenantapp.KolAssistJob, model string, meteredOutputText string) {
 	resourceType := "kol_assist_task"
 	resourceUID := job.TaskID
-	if err := tenantapp.CompletePendingAIPointsByResource(ctx, w.pool, job.TenantID, resourceType, nil, &resourceUID, model); err != nil && w.logger != nil {
+	if _, err := tenantapp.CompletePendingAIPointsByResource(ctx, w.pool, w.cache, job.TenantID, resourceType, nil, &resourceUID, model, meteredOutputText); err != nil && w.logger != nil {
 		w.logger.Warn("kol assist ai points confirmation failed",
 			zap.Error(err),
 			zap.String("task_id", job.TaskID),