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/repository/ai_point_usage_repo.go

@@ -52,7 +52,7 @@ type AIPointUsageListParams struct {
 
 type AIPointUsageRepository interface {
 	Create(ctx context.Context, input CreateAIPointUsageLogInput) (int64, error)
-	MarkCompleted(ctx context.Context, tenantID, id int64, model *string) error
+	MarkCompleted(ctx context.Context, tenantID, id int64, requestChars, baseChars, points int, model *string) error
 	MarkRefunded(ctx context.Context, tenantID, id int64, errorMessage string) error
 	FindPendingByResource(ctx context.Context, tenantID int64, resourceType string, resourceID *int64, resourceUID *string) (*AIPointUsageLog, error)
 	List(ctx context.Context, params AIPointUsageListParams) ([]AIPointUsageLog, error)
@@ -97,18 +97,21 @@ RETURNING id
 	return id, err
 }
 
-func (r *aiPointUsageRepository) MarkCompleted(ctx context.Context, tenantID, id int64, model *string) error {
+func (r *aiPointUsageRepository) MarkCompleted(ctx context.Context, tenantID, id int64, requestChars, baseChars, points int, model *string) error {
 	_, err := r.db.Exec(ctx, `
 UPDATE ai_point_usage_logs
 SET status = 'completed',
     model = $1,
+    request_chars = $2,
+    base_chars = $3,
+    points = $4,
     error_message = NULL,
     completed_at = COALESCE(completed_at, NOW()),
     updated_at = NOW()
-WHERE id = $2
-  AND tenant_id = $3
+WHERE id = $5
+  AND tenant_id = $6
   AND status = 'pending'
-`, model, id, tenantID)
+`, model, requestChars, baseChars, points, id, tenantID)
 	return err
 }