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/tenant/app/question_expansion_service.go

@@ -317,7 +317,8 @@ func (s *QuestionExpansionService) GenerateByAIDistill(ctx context.Context, bran
 		candidates = append(candidates, buildCandidate(text, QuestionSourceAIDistill, false, existing, brandCtx))
 	}
 
-	if err := CompleteAIPoints(context.Background(), s.pool, actor.TenantID, *reservation, result.Model); err != nil {
+	completed, err := CompleteAIPoints(context.Background(), s.pool, s.cache, actor.TenantID, *reservation, result.Model, result.Content)
+	if err != nil {
 		return nil, response.ErrInternal(50127, "ai_points_commit_failed", "failed to confirm ai points")
 	}
 
@@ -329,7 +330,7 @@ func (s *QuestionExpansionService) GenerateByAIDistill(ctx context.Context, bran
 
 	return &QuestionCandidateResult{
 		Candidates:      candidates,
-		AIPointsCharged: reservation.Points,
+		AIPointsCharged: completed.Points,
 	}, nil
 }
 
@@ -413,10 +414,11 @@ func (s *QuestionExpansionService) FillQuestionCombination(ctx context.Context,
 	}
 
 	filled := sanitizeQuestionCombinationFillResult(payload, fallback)
-	if err := CompleteAIPoints(context.Background(), s.pool, actor.TenantID, *reservation, result.Model); err != nil {
+	completed, err := CompleteAIPoints(context.Background(), s.pool, s.cache, actor.TenantID, *reservation, result.Model, result.Content)
+	if err != nil {
 		return nil, response.ErrInternal(50127, "ai_points_commit_failed", "failed to confirm ai points")
 	}
-	filled.AIPointsCharged = reservation.Points
+	filled.AIPointsCharged = completed.Points
 	if s.cache != nil {
 		cached := filled
 		cached.AIPointsCharged = 0