feat(admin): admin grant — raw benefits and by-product reward bundles

The /_gm user card gains a Grant panel: grant raw benefit atoms (hints /
no-ads days / forever) or a defined value product (a reward bundle, including
an archived one), origin-picked. Both write an admin_grant ledger row via
payments.Grant / GrantProduct; the by-product grant records the source
product_id + snapshot. Both refuse a chips atom (never grant currency) or a
tournament atom (no credit target yet); chips/tournament products are also
kept out of the by-product picker.

Tests: the console grant end to end (raw, by-product, refuse a chips pack,
CSRF-guarded).

backend/internal/payments/service.go

@@ -156,6 +156,41 @@ func (s *Service) Grant(ctx context.Context, accountID uuid.UUID, origin Source,
 	return s.store.grant(ctx, accountID, origin, d, snapshot, s.clock())
 }
 
+// GrantProduct grants a product's benefit atoms (hints, no-ads days) to an origin as a zero-price
+// admin sale, recording the source product on the ledger row (auditable to it). It refuses a
+// product carrying the chips atom (never granted — D16) or the tournament atom (no credit target
+// yet), and one whose atoms yield no grantable benefit.
+func (s *Service) GrantProduct(ctx context.Context, accountID uuid.UUID, origin Source, productID uuid.UUID) error {
+	if !origin.Valid() {
+		return fmt.Errorf("payments: invalid grant origin %q", origin)
+	}
+	in, err := s.store.productInput(ctx, productID)
+	if err != nil {
+		return err
+	}
+	var d benefitDelta
+	for _, a := range in.Atoms {
+		switch a.Atom {
+		case "hints":
+			d.hintsAdd += a.Quantity
+		case "noads_days":
+			d.noAdsDays += a.Quantity
+		case atomChips:
+			return ErrCannotGrantChips
+		case "tournament":
+			return ErrCannotGrantTournament
+		}
+	}
+	if d.zero() {
+		return ErrNothingToGrant
+	}
+	snapshot, err := marshalGrantProduct(productID, in.Title, d)
+	if err != nil {
+		return err
+	}
+	return s.store.grantProduct(ctx, accountID, origin, productID, d, snapshot, s.clock())
+}
+
 // MergeTx merges the secondary account's segments and benefits into the primary inside the
 // caller's transaction (the account-merge flow). The caller invalidates the affected caches
 // after committing (Invalidate).
@@ -244,3 +279,20 @@ func marshalGrant(d benefitDelta) ([]byte, error) {
 	}
 	return b, nil
 }
+
+// marshalGrantProduct builds the snapshot for an admin grant-by-product: the source product and the
+// benefit atoms it granted (price 0).
+func marshalGrantProduct(productID uuid.UUID, title string, d benefitDelta) ([]byte, error) {
+	atoms := map[string]int{}
+	if d.hintsAdd > 0 {
+		atoms["hints"] = d.hintsAdd
+	}
+	if d.noAdsDays > 0 {
+		atoms["noads_days"] = d.noAdsDays
+	}
+	b, err := json.Marshal(purchaseSnapshot{ProductID: productID.String(), Title: title, Atoms: atoms, PriceChips: 0})
+	if err != nil {
+		return nil, fmt.Errorf("payments: marshal product grant snapshot: %w", err)
+	}
+	return b, nil
+}