feat(admin): grant hints to a user's wallet from the console

Add an "Add hints" form on the admin user card that additively tops up a
player's hint wallet (1-100 per grant). The grant is raise-only by
construction (an additive UPDATE never lowers the balance) and stays correct
under a concurrent in-game spend; a per-grant cap bounds a fat-finger, since
the console can never reduce a wallet.

The in-game hint policy is unchanged and already correct: a game offers the
per-seat allowance plus the wallet, spending the allowance first and the
wallet only after (covered by TestHintPolicy).

backend/internal/account/account.go

@@ -430,6 +430,30 @@ func (s *Store) SpendHint(ctx context.Context, id uuid.UUID) (bool, error) {
 	return n > 0, nil
 }
 
+// GrantHints adds n hints to the account's wallet and returns the new balance. n must be
+// positive: the additive update can only raise the balance, never lower it, so it enforces the
+// admin console's raise-only rule by construction and stays correct under a concurrent SpendHint.
+// It returns ErrNotFound when no account matches.
+func (s *Store) GrantHints(ctx context.Context, id uuid.UUID, n int) (int, error) {
+	if n <= 0 {
+		return 0, fmt.Errorf("account: grant hints %s: n must be positive, got %d", id, n)
+	}
+	stmt := table.Accounts.
+		UPDATE(table.Accounts.HintBalance, table.Accounts.UpdatedAt).
+		SET(table.Accounts.HintBalance.ADD(postgres.Int(int64(n))), postgres.TimestampzT(time.Now().UTC())).
+		WHERE(table.Accounts.AccountID.EQ(postgres.UUID(id))).
+		RETURNING(table.Accounts.HintBalance)
+
+	var row model.Accounts
+	if err := stmt.QueryContext(ctx, s.db, &row); err != nil {
+		if errors.Is(err, qrm.ErrNoRows) {
+			return 0, ErrNotFound
+		}
+		return 0, fmt.Errorf("account: grant hints %s: %w", id, err)
+	}
+	return int(row.HintBalance), nil
+}
+
 // FlagHighRate stamps the soft "suspected high-rate" marker with at, only when
 // the account is not already flagged — the first sustained episode wins, and a
 // re-flag after an operator clear starts a fresh timestamp. An infra marker, not